I seem to have found this in The Grounds... It appears to be a stone canine head, with textureless white ears and hair. (I have a texture pack mod installed so that is why the rock texture looks different.)
(https://i.ibb.co/cbGLnCt/screenshot-01312021-000229593.png)
I have nametags on and this dude has no nametag (nor is it moving at all), so it is not a player with a preset who glitched into the wall.
More proof it is not a player: One ear is folded and another is normal. I do not have a mod that turns one of the ear models that way.
Is this supposed to be a secret of the game? OR... Could this be teasing something? 🤔
Ah you found one of the few heads!
There are Easter eggs all over the maps - it makes a fun day to try to find them all!
Well, shave me down and call me a sphynx. I guess I better look for the rest!