The Firearm Blog often has interesting home made pieces, but this is amazing. Or hideous, depending on what side of the brain you are examining it with. The maker of this was using both.
Personally I’d name it the obvious, the Bullpup Mutt. You can see that he started with an H&K SL8 polymer thumbhole stock, and cut into it and added a plate over the side of it. There is a STEN bolt in a tube-gun receiver, with a FAL return spring. I’m not sure how that works with the existing STEN bolt spring shown in one of the other photos on The Firearm Blog post.
Rat bikes are essentially bikes cobbled together with various other makes and models to form a unique functioning transport. This is certainly a Rat Gun.