Rock jellies. Before I hit T3 I had only ever HEARD of the things. Nobody seemed to know anything about them, except for telling me they supposedly spawned in place of stone blocks.
Well, I finally completed my set of 4* gear and jumped into a T3 dungeon, one of those Cravat Hall type of levels, with an undead theme.. this one specifically was called something "Gallery".
I go into one room, a rock jelly spawns alongside of a regular T3 jelly. Later on, in the same level, I go into another room and TWO of the rock jellies spawn together (along with a gun puppy and a couple kats). Oh, also I saw a Moustache jelly but.. the poor thing decided to spawn directly next to a perpetually-exploding-and-respawning line of explosive blocks and immediately died, otherwise I would have spent some time just pushing it around.
Anyways, that was a total of three rock jellies (and a mustache jelly) all in a single dungeon floor. Either that was a pretty extraordinary bit of luck, or... these things are just common up here in T3?
Rock Jellies are common in Grim Gallery, But anywhere else in the game they spawn in place of Blocks.