Magma Beasts were large, humanoid predators native to the deeper caves of Androzani Minor.
They looked like a hybrid between dinosaurs and armadillos and had massive, fanged heads, armour-covered bodies and cape-like shells. Magma beasts came up to the higher caves when they were hungry, eating the soldiers from Androzani Major who were common in the area. They frequently encountered Sharaz Jek's androids, but didn't bother them since they only ate meat.
The Fifth Doctor encountered a magma beast while searching for Peri Brown. It chased him, but was distracted by one of Jek's androids. (TV: The Caves of Androzani)