Axolotl

The perpetually smiling salamander that can regrow its own brain/Small Animals

The axolotl (Ambystoma mexicanum) is a neotenic salamander that retains its larval features throughout its life, including external gills that look like feathery crowns. Native only to Lake Xochimilco in Mexico City, it can regenerate limbs, spinal cord, heart tissue, and even parts of its brain — abilities that make it invaluable to medical research. It is critically endangered in the wild but thrives in laboratories and aquariums worldwide.