Beatnik mutants that inhabit Maas Neotek. They all have unique names based on beatnik writers, and different hepcats have different stats and equipment. They range from pitifully easy to insanely difficult, although all of them give the same ammount of XP. Hepcats have a contract, and it can be quite lucrative since their xp yield is good and you can salvage weapons and armor worth a fair amount from them. You shouldn't try until you can defeat Arachnos.
Note: There's a quest you can do that has you nuke Maas Neotek. Immediately after it is nuked the city is radioactive, and Hepcats won't spawn for a very long time. If you go to Maas Neotek and nothing is there, that's probably what happened.
Every Hepcat is unique, so your strategy should be different depending on which one you're fighting. They also aggro at different rates. Some of them won't aggro at all, some wait a bit, some aggro when another hepcat aggros, and some aggro immediately. Here's a run down of hepcats by name: (Not comprehensive, yet)
Keasey: Simple. Keasey doesn't have any weapons, so he should be a cinch. Delayed aggro.
Ricky: Medium. He uses a Fine Wakizashi, and attacks quickly.
Butch: Medium. Uses a Slam Bat, and can break limbs with a good hit. This is a bad thing.
Lucy: Medium/Hard. She uses a Sonic Scalpel, which bypasses all soaks and does bleeding damage. She attacks quickly, and will most likely kill you if you don't take her out in the first few turns. Aggros immediately.
Ginsberg: Really tough. Ginsberg is fast and hits hard with his Gamma Fist, causing frequent knock outs. Ginsberg won't aggro immediatly, so you can normally pass through a room he occupies unharmed. Watch out if he's in a room with an auto-aggro hepcat like Jack, however.
Burroughs: The toughest hepcat. Run. Run away. Burroughs won't leave his location, so you can move in and out as you need to heal. He drops an adamantium cock sock if you kill him.