Random Permanent Rock Lobster Follower

No, there's just an old bug that you sometimes see when going between the layers (Caves, Ruins, Surface). It's very rare, but more common if you spam enter / leave, I've personally seen it maybe 4 times since Caves were introduced?

Anyway, the list of perma-befriended follower mobs that can spawn this way are: Rock Lobsters, Pigmen, Bunnymen, and surprisingly Beefalo. (Note, Beefalo could spawn this way before domestication was a thing, so presumably it's from the horn blowing mechanic?) They don't stop being befriended to you. Over on the Subreddit, we get a post about it at least once every three months if I had to guess? (It definitely has some good utility as a bug, but it's way too inconsistent, any scenario it'd be useful in is very niche, and the bad outcome, Rock Lobsters outweighs any good potential in my opinion.) 

There is no fancy set-up, it's just random chance when exiting / entering the Caves.