Was the Greek Herakles the same hero that the Romans worshiped as Hercules? The answer devolves from Yes to Maybe and then No.
The differences between the Greek and Roman gods and goddesses have become so blurred over the centuries that it is sometimes difficult to tell one from the other. Instead of comparing the Greek and Roman deities, many books and mythology web sites use the term "equivalent" as if the Greek and Roman deities were identical. There are numerous popular translations of ancient Greek literature in which the Greek gods and goddesses are never called by their Greek names! Zeus becomes Jove, Ares becomes Mars and Herakles becomes Hercules etc. The translators substitute the Roman names for the Greek names and no one seems to notice or care. The result has been that many people are convinced that the ancient Romans did not have an original religion at all and that the essence of their spiritual life was nothing more than a revival of the ancient Greek religion with the addition of a few Roman flourishes and new Latin names for the gods and goddesses. Both religions have been relegated to the category of myth and today it would seem that to quibble over their unique differences is unimportant. The ancient religions of Greece and Rome were not myths to their worshipers ... the real myth is that we equate these two religions so willingly and completely.
One of the main problems with the heritage of the Roman deities is that there is no Theogony to document their interconnections. To refresh your memory, Theogony is a poem written by the Greek poet Hesiod in the mid-seventh century BCE in which he methodically lists the Greek gods and goddesses, their parents, their descendants, their role in the cosmos and, most importantly, their true names and pseudonyms. Sadly, our knowledge of the ancient Roman religion does not include a definitive text such as Theogony to delineate their gods and goddesses. I believe that this gap in our knowledge is one of the primary reasons we so easily confuse these two ancient religions.
It's true that the Greeks and the Romans saw forces of nature as the personifications of divine beings. It's also true that they both believed that their gods and goddesses could assume the guise of mortal humans. From these basic beliefs the Greeks and Romans each built an elaborate and distinctly different pantheon. The two pantheons were similar in many ways but not, as some would have you believe, the same gods and goddesses with different names. If we consider the difference in time, the difference in language and, most importantly, the difference in culture which separated these two civilizations, it should be clear that it would be impossible for them to have the same exact religion. Similar, yes. Identical, no.
There is a modern example of this concept that makes the differences between the Greek and Roman religions seem like night and day. In the Western Hemisphere, the most popular religion is Christianity. This religion is based on the Bible. Over the centuries, the Christian faith has splintered into numerous sects with different interpretations of the Bible. Some aspects of the Christian religion have diverged from their common heritage so dramatically that there have been schisms within some of the sects creating sub-sects that could easily qualify as cults. If the SAME religion can be splintered into dozens of different sects with drastically different belief systems, is it so hard to accept the probability that two SIMILAR religions, such as the Greeks and Romans practiced, could be distinctly different?
Most myth researchers are quick to say that the Greek Herakles was taken into the Roman pantheon directly from the ancient Greek religion and that, aside from the spellings of their names, there is no inherent difference between the Greek Herakles and the Roman Hercules. However, when we get to the details of his existence, the Roman Hercules becomes more and more Roman and less and less Greek.
The story of Cacus and the Roman Hercules is an example of the way in which the Romans incorporated the Greek Herakles into their pantheon and simultaneously made him a Roman. When the Roman Hercules was returning from Taking the Cattle of Geryon (Geryones), which was his Tenth Labor, he encountered the half-human monster, Cacus, and slew him. Not surprisingly, Cacus lived in Italy on the Tiber River. He was the son of Vulcan, the Roman god of creative fire. Displaying the characteristics of his father, Cacus was gigantic and belched fire and black smoke. After the Roman Hercules had taken Geryon's cattle, he camped near Cacus's cave. Unable to resist any form of criminal behavior, Cacus stole four bulls and four heifers from Hercules's herd. Cacus must have been familiar with the Homeric Hymn to Hermes because he used the same trick Hermes had used when he stole the cattle of Apollon, i.e. he marched the cattle backwards into their hiding place so that their hoof prints would deceive anyone who tried to find them. Although well hidden, the stolen cattle began to bellow and Hercules soon found Cacus's cave. After a thunderous battle, Hercules pulled the lifeless body of Cacus from the shattered cave, reclaimed his cattle and restored the accumulated plunder to the local inhabitants. This story is from the Aeneid (book 8, lines 255-410) and perfectly illustrates the way in which the Romans borrowed from previously tried-and-true legends to make their own beliefs seem more credible. It should be noted that the Greek Herakles did not encounter Cacus when he took the Cattle of Geryon.
Other lessons can be learned from the Aeneid about how the Romans disdained the ancient Greek religion and, in fact, did not adopt or admire the Greek gods, goddesses or traditions. The Aeneid was written by the Roman poet, Virgil, between the years 29-19 BCE during the reign of Augustus Caesar (a.k.a. Octavian). The Aeneid was an undisguised attempt to re-instill the noble values on which Rome had been founded and to give new faith to the people of Rome after the flagrant excesses of Julius Caesar and Marcus Antony. The Roman gods and goddesses portrayed in the Aeneid were not the same Immortals that the Greeks worshiped. The Romans were a different breed and so were their deities. In the Aeneid, Greek heroes, such as Menelaos (Menelaus) and Diomedes, are depicted as butchers and cowards. Graceful Greek goddesses, such as Iris and Pallas Athena (Athene), were given a dark countenance that was both unflattering and semi-evil. The Roman goddess of Love, Venus, was no Aphrodite. She was not just a manipulator like Aphrodite, she was a trickster in the most vile sense of the word. The Aeneid was not a celebration of the religion the Romans inherited from the Greeks. The Aeneid was a denunciation of Greek religious values and its institutions.
After his adoption into the Roman pantheon, the Roman Hercules was given an alter in Rome and it's been suggested that he replaced a less popular Roman tough-guy named Recaranus. The idea of a righteous strong-man or tender-hearted tough-guy seems to have always been a popular standard in Western thought and the Romans were quick to adopt the Greek Herakles and give him a Roman name, distinctly Roman attributes and a Roman father.
The Greek Herakles was the son of Zeus and Alkmene (a mortal woman). He lived and died a generation before the Trojan War and fathered scores of children. His descendants ruled many cities and regions of the Greek world for hundreds of years after his death. The cult of Herakles was popular throughout the prehistoric and historic Greek world. Greek historians such as Herodotus and Xenophon wrote of Herakles as if he was a flesh-and-blood man and made no apologies for their sincere beliefs.
In the ancient Roman religion, Jupiter is the father of Hercules. No matter how you bend and stretch the facts, Zeus and Jupiter can not be equated. The best argument I've seen which attempts to equate Zeus and Jupiter is strictly semantic and not very convincing. This argument assumes that Zeus's name was derived from the Indo-European word Dyeus which means Sky. It would follow that Jupiter's name was derived from Dyeu-Pater which might be read as Sky-Father. This argument adds assumption to assumption and seems to unravel when the same logic is applied to the Roman names of other gods and goddesses. How did Poseidon become Neptune or how did Athene become Minerva. It seems to me that if we are trying to make an Indo-European connection between the names for the Greek and Roman deities and declare that the names are equivalent, the correlations should apply across the entire spectrum of names and not just to one name. If we want to be as generous as possible, the correlations should apply to at least two or three names and not just one belabored example.
If we rely solely on the historical record we have more evidence that the Greek Herakles was forced into the Roman pantheon and not a natural part of it. The earliest date for the founding of Roman culture (not Rome itself) is circa 750 BCE. When the Trojan War was fought (circa 1250 BCE) Herakles had been dead for one generation. That would mean that Herakles interacted with the Roman deities 500 years before they were instituted. This would mean that the Roman Hercules died 500 years before his father was conceived.
There are many convincing arguments which theorize that religious beliefs have evolved along the same geographic paths as technology and culture. These theories assume that ideas have generally migrated from the east to the west. If true, that would mean that the Egyptian religion had elements of the Sumerian and other Asian religions. In turn, the Greeks borrowed from the Egyptians. Likewise, the Romans used ideas from the Greek pantheon to create their own religion. With every step along the evolutionary road, religion, technology and culture have become more sophisticated. The Roman religion was thus more mature and refined than the ancient Greek religion and there are historical instances where the Romans banned certain Greek religious customs and traditions because they seemed barbaric to Roman sensibilities. The Bacchanalia is one example. The Bacchanalia was a festival in honor of the god Bacchus (a.k.a. Dionysos, god of Wine). It was fueled by wine and intended to incite frenzy in the participants. The Romans banned this longstanding Greek festival in the second century BCE because they considered the festivities to be of a depraved and wanton nature. Please consider what it might take to offend Roman sensibilities in those ancient times. Slavery, crucifixion and gladiatorial combat were common fare in Rome. The subjugation of the known world did not offend the Romans, why would wild revelry and drunkenness offend them? Perhaps it was not the behavior of the worshipers but rather the god which the Bacchanalia honored that irritated the Romans. The Romans did not preserve and perpetuate the ancient Greek religion, they banished it.
Another example of how the Greek and Roman names have become interchangeable and confused might come from the world's first historian, Herodotus. When Herodotus was traveling in Egypt and Asia Minor, he repeatedly referred to the local deities by their Greek names. He states that he visited the temple of Zeus and Ares and a host of other gods and goddesses with Greek names while he was in those foreign lands. What he was actually saying was that the Egyptians, Scythians, Babylonians and so forth, worshiped deities which reminded him of his own gods and goddesses and he simply used the Greek names for these foreign deities to make it easier for his Greek audience to appreciate what he was describing. This is essentially what has happened to the Greek and Roman gods and goddesses, i.e. for the sake of convenience, not accuracy, the names of the deities of the two pantheons have been interchanged and, with the passing of time, the differences between the Greek and Roman gods and goddesses have become indistinguishable to the casual observer.
When the Greek Herakles was inducted into the Roman pantheon, his character and attributes were modified to suit the firmly established Roman culture. The underlying traits which made him such a powerful influence on the ancient Greek world were transplanted into the new fertile mythos of Rome and, as with any belief system, did not remain static but transmuted the old, dead Greek Herakles into the new, living Roman Hercules. The hero is dead, long live the hero.