A llama which "wears" the texture of the inventory GUI spawned out of nowhere. Obviously, it spawned in a mountain biome. I believe this bug cannot be reproduced/recreated, though it might possible for other players to encounter this bug.
I didn't use any mods, resource packs, and datapacks while playing the game; it's just your regular vanilla Minecraft game (1.16.3, also other 1.16 versions too I think)
(This bug makes me laugh and worry at the same time 😂)