Werewolves are usually found with Grey Wolves, Giants, and Grey Imps. They're only found in the first half of the game.
Like most things that can poison you, I'd say just don't fight these at all.
Werewolves are the same in every version. Werewolves are traditionally men with the ability (or curse) of turning into a half-man, half-wolf creature (and sometimes to a full wolf), often with the full moon. The werewolf's bite is supposed to spread the condition to the victim, hence the poison status in this game. The name comes from the Old English word "werwulf" ("wer" meaning man, "wulf" meaning wolf). The condition of being a werewolf (or other were-creature) is called Lycanthrope. This comes from "lycos" meaning wolf, and "anthropos" meaning man. Werewolves can also be found in Dungeons & Dragons, Dragon Warrior, and the Castlevania series, as well as many other things. Werewolves later appear in FF3 & Mystic Quest, but for more info, see the Wolf page.