Most people having suggestions for the nether seem to want it to be harder, I'm thinking the opposite. The nether should be able to be liveable for a long time, so why not have a passive mob which drops food?

My suggestion - the Kangaroo. I know its not in classic nether "zomg make it a skeleton thing that kills you by looking at you" style with fire and lava chucked in, but the nether is "down under", so i feel that it's an appropriate mob. It would also be a fun mob to have in the game.

Given the inability of Kangaroos to be herded and kept by farmers IRL, I'd say that the kangaroo should be a 'hunt only' sort of passive mob. They would be able to jump 2 blocks high meaning that you couldn't fence them. They also could not be bred.

The downside to this idea is that it is sort of redundant. I had it a while back before mobs could be pushed through portals. Still, I think new mobs are always exciting and it certainly feels like a fun mob to make the nether a little more interesting.