The Apex Mutant seems to be a failed experiment that can be found in big Apex Labs. The Mutant Apexes are also sometimes found within Glitch-like cages or cells. Also, in certain mushroom camps, a green apex(possibly mutant) can be seen in a pod. The one pictured below is within a cell adorned with Glitch Flagpoles, Glitch Chests, and bars. Nearby is a wheelbarrow.
They may or may not attack the player or the NPCs directly, but if a player touches them it inflicts a huge amount of damage well above the average damage of enemies of the world's level. The NPCs in the Lab will not attack the Mutant Apexes.