Other video games, fantasy art, comic book art, old cartoons, classical paintings, etc.
I look at a lot of older games all the way from Amiga to Sega Genesis to Arcade.
I also break down games, record videos, analyze animation frames in photoshop, study sprite sheets, and do experiments and sketches.
I study whatever games I think mine will play like. There's quite a long list because each game has some aspect that I like. For example, maybe I like the simple walk animation in one game, or maybe I like the sword play mechanics in a game. I use them for reference, not to copy, but to have a starting point.
I'll list a few I guess:
samurai shodown (arcade)
golden axe series (arcade)
shadow of the beast (amiga and genesis)
altered beast (genesis)
revenge of shinobi (genesis)
most old fighting games (x-men vs street fighter, king of fighters, etc.)
kid chameleon (genesis)
ranger x (genesis)
super castlevania iv (snes)
symphony of the night (ps1)