Brain Slime

The Brain Slime is a hostile monster added by Primitive Mobs. It spawns in Beaches.

It acts similarly to Slimes, hopping around towards players, but will also jumps towards animals if they are small enough (size 1 or 2). When near its target, it jumps at its target's head and rides it, periodically dealing damage until the target dies or hits it. This damage can be prevented by wearing a Helmet, in which case the Helmet will take damage instead.

After dealing damage 10 times, the Brain Slime will hop off its target and grow a size larger, with no upper bound. A Brain Slime's health is equal to its size squared, and its damage is equal to 1.25 times its size.

Just like Slimes, tiny slimes (size 1) drop 0-2 Slimeballs on death, while larger one split into 2-4 Brain Slimes with half the size (rounded down).

Trivia

 * The Brain Slime originally comes from the beta 1.7 mod Brain Slimes.