The Iosaur is an aggressive melee mob that spawns in Precasian Underground Dens. It is a hunter mob that can only be damaged if the player is level 52 or higher, and grants 200 hunter experience points when killed. It regenerates health very rapidly (about 30 (×15) health per second), and also has high knockback as well as knockback immunity. This mob applies Slowness to the player on hit, which lasts for 2.5 seconds.
- It slightly resembles the Moschops, a prehistoric animal which wasn't an actual dinosaur.
- It strongly resembles the Spinoledon.