Name: Hercules (imposter)
First Appearance: The Avengers (vol. 1) #10 (Nov. 1964): "The Avengers Break Up!"
Number of Appearances: 1
Worked for: Immortus
Race: Space Phantom
Hercules is one of the great hero figures from ancient myth. Eventually he would be adopted into the Marvel Universe as a full-fledged character, complete with his traditional Greco-Roman mythological origins.
But before this modern Marvel version of Hercules appeared, there was the first appearance of Hercules in the Marvel Universe was way back in Avengers (vol. 1) #10, when the time-traveling villain Immortus plucked various figures from history in order to have them battle the Avengers. One of the figures Immortus brought into modern times was Hercules. Immortus had Hercules battle Thor (appropriately enough, given that Thor was also a favorite son/deity/hero figure from an ancient pagan pantheon).
But Hercules' appearance in Avengers #10 was rather out of character with his future appearance in the Marvel Universe, and the fact that he had this battle was subsequently completely ignored in later stories that "re-introduced" him to the Marvel Universe, e.g., Journey into Mystery Annual #1 (1965). To explain these discrepancies, an explanation was provided much later (in the 1998-1999 12-issue mini-series Avengers Forever: The "Hercules" who appeared in Avengers #10 was actually a Space Phantom in disguise. (Space Phantoms are a race of aliens who can take on the for of other living beings, temporarily shunting those beings to a Limbo dimension.) Other parts of this story (and other stories) more firmly established the Space Phantoms as allies of Immortus. So "Hercules" in Avengers #10 is based on the "real" Marvel Hercules (the Greco-Roman deity), and may in fact have directly stolen his form from the real Hercules, but he was not, in fact, the actual Hercules. He was an imposter.
For a while it was thought that this Hercules imposter was actually a Dire Wraith (a member of another alien that can copy the forms of other beings), but the Avengers Forever series firmly established (via retcon) that it was a Space Phantom.
Suggested links for further research about this character and the character's religious affiliation: