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).
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.