Vicious! Cruel! Ruthless! That's what these beasts are. They attack anything and everything until they've ripped it to shreds, all the while smiling that sadistic grin...In-game description
The Monster is the Stage 3 power-balanced form of the Ruin group.
Monsters were removed from the game before it was released, so although their data still exists, they do not appear anywhere.
A Monster should evolve from Minimon, but it is disabled in the alpha version. It does not evolve further.