Alright, here goes…
Name: Lurker
Hostility: Hostile (deals 4 health points of damage or 2 hearts)
Special Abilities: Camouflages with all solid blocks.
Biome: Caves. They’re a terrain feature and not a biome though.
Behavior: Usually, lurkers wander around, blending in with nearby blocks. When they detect a player, they quickly stop camouflaging with nearby blocks and they attack the player. They only have a melee attack, which they perform by launching themselves at the player. Lurkers also camouflage with blocks if they’re damaged. (When not camouflaged, lurkers resemble the appearance of the last block they blended in with)
Health Points: 20 (10 hearts).
Rough Sketch: 
