Imps are usually found together in groups of 3-5 but may also be with Wolves, Grey Imps, and Grey Wolves. They will be the first enemy you fight. You may think them to be the weakest enemy in the game but that distinction belongs to the Pirate. Pirates do, however, have higher attack power. For the most part, you can only find imps in the very beginning of the game, but due to a small bug in the enemy mapping there are a couple of other places they show up.
What? You don't need strategy to fight imps! :) Well, I guess the best thing is to spread your attacks out, because any class can kill them in one hit. If you have them to spare, a spell that attacks everything can wipe them all out at once. If you're too strong to bother, you may wish to just save time by running away. Also, being in the Giant category, the Giant Sword (Great Sword) works better against them in the WSC version. Of course there's absolutely no reason you'd want to bother. :)
In the original Japanese version (and FF Origins) an Imp is called a Goblin. Goblins are mean little humanoids that live in caves. Like a lot of the enemies, Square probably got goblins from Dungeons and Dragons, but goblins have been around a lot longer than that. The word comes from the Greek kobalos meaning rogue. They return in at least Final Fantasy 2-7, and 9 as well as Tactics, Mystic Quest, and FF Legend. You can also find goblins in things like Magic: The Gathering and Warcraft 2. As for the name Imp, I don't know what the thinking was here. "Goblin" would've fit easily, and they aren't exactly synomyns. Anyway, an imp is a small demon, and they even appear in FF2 (which was out long before FF1 was released in English, so I'm especially bothered by this) and are coded into FF4, but don't actually appear in the game. Imp is also a status ailment in the US version of FF6, but it's called Kappa (a Japanese river monster) in the Japanese version.