yea, someones missing the mob's model.
What addons do you have? That's a very nice and clean look.
somehow the model just didn't load, and that's what that is, a placeholder for a missing model, don't know how it happened, maybe the server send the wrong model id or something but some how it happened.I wasn't because he was grinding in that cave and I already fought that same monster multiple times. So I'm not sure what happened.
One of the TukUI versions judging by the look of it.
Look at Tukui > Blog and find your poison there (ElvUI is a famous one for example and can be updated trough the curse client).