Monster somehow glitched through the terrain on this spot of the map and got himself stuck.
Here we see early concepts for what became the Behemoth.
How he got stuck in that rock, I fail to understand.
We domed him in the area, trapper was standing on the rock in question, monster tried to hit him with a jump + attack from the base of the rock and just glitched inside.
I think I can get a video also ,since trapper is a streamer friend of mine. It will be up today in the evening I think.
EDIT: Here is the video