While building on a new world, I noticed that an Axolotl was beating up a Glowing squid on the island I was building on. The biome type was a mushroom fields, and I know that only bats and mooshrooms spawn in the biome.

The attachment below shows the Axolotl that I found. I couldnt get a picture of the glowing squid though, as the Axolotl had killed the squid before I could get a screenshot.