Playing Minecraft (demo mode) and what's this thing?

I just found out about Minecraft's demo mode, so thought I'd try it out. I found a nearby cave, explored it and put some torches in, and… Started hearing this growling thing. I checked and there aren't any creatures nearby, just some sheep several dozen blocks away (Block = 3 feet?) so… What IS that?

Maybe a wolf considering your playing demo.

Growling? Or snarling? Zombies make a snarlish sound and so do spiders.

More specific please