Can someone explain this to me?

So I booted up rust to find that the building that normally occupies the menu screen was gone

This was revealed

If anyone can explain this, that would be an interesting explaination

It was a capture of Garry dancing naked on NYE.

Replacement for zombies possibly?

Looks like a warebear

looks just like a bear standing up. I believe they put this in to cast the shadow you see moving in the doorway. Not sure why they used a bear though instead of a person. Maybe because they already have a movable model for bears.

bears are being updated over time to stand up when they attack to claw you, this is just a bug where the bear is doing the standing animation ready to attack