The gorilla is named after creatures that were probably not gorillas.
Around 500 BCE, Hanno, a Carthaginian navigator, led a fleet down the west coast of Africa. His account survives in a Greek translation. Near the end, the fleet reaches an island in a lake, full of "savage people." Most of them were women with hairy bodies, "whom our interpreters called Gorillae." The sailors caught three women, who fought so fiercely with teeth and hands that the sailors killed them. They carried the skins back to Carthage. Hanno reportedly had the account displayed in a temple of Baal Hammon there.
Nobody knows exactly what Hanno saw. Etymonline says the creatures are now thought to have been chimpanzees. It calls gorillai "allegedly an African word," so even the language behind the name is uncertain. Yale Alumni Magazine mentions one candidate, Fulani gorel, "little men."
The word then sat unused for more than two thousand years. In the 1840s an American missionary, Thomas Savage, obtained skulls and bones of a large ape in Gabon. Yale Alumni Magazine reports that he never saw a living one. He relied on local hunters for a description of the animal.
Savage wrote to Jeffries Wyman, a Harvard anatomist, and the two co-published the first scientific description. Etymonline dates the English word to 1847, when the ape was named Troglodytes gorilla. Savage had hoped the local Gabon name, enge-ena, would be used. Etymonline credits Savage with the name, but Yale Alumni Magazine says Wyman chose "gorilla," recalling Hanno's word for wild men.
So a Greek spelling of a possibly African word, for animals that were possibly chimpanzees, became the name of a different ape. Etymonline says that from 1884 the word was also used for people thought to look or act like one.
The gorilla is often confused with the guerrilla, but the two words are unrelated. Guerrilla is Spanish for "little war." It entered English in 1809, during the Peninsular War.