Steps to reproduce:
1. Put a cat or dog in a minecart (optionally flow water on to it, to maximise the animal's life)
2. Move at least 18 blocks away
3. Jump in lava (or create a line of lava, jump in, and swim in it until 18 blocks away).

The animal starts burning. Doesn't happen with any other mobs I tried (ocelots, wolves, skeletons, horses (wild + tamed), villagers).