They can only occur in the Skull Cavern, and the predictor will check up to level 500 for them. Dinosaur levels were also added in 1.4 these have a grassy tileset and are primarily inhabited by Pepper Rex.
These can only happen if the quarry itself has been unlocked and will override other types of infestations, so the predictor only calculates them if the save has quarry access.
This calendar showss the prediction of various special levels in
