salamander? fuck that thing sounded weird, a bit like a weird pig. it was pretty tough too, 130 hp. i encapsulated it in dirt and gave myself a small opening to attack with my wooden yoyo from. it's attacks appear to be ranged.
addition: pretty sure they spawn in normal underground. in the cavern layer.
addition number 2: drops are unknown save for coins. heard another one, i'll be seeing if i can't find and kill it to get whatever special loot it might have.