It's a descriptive word for a small inbred hamster/chincilla/armadillo or a mix of the three. Usually these creatures are dirty, quite often welsh and look like a disfigured pigs butt.
If you were to actually see a Manchillo you would go into shock and if you touched one you would be very ill as a lot of them carry herpies.

when you've done something so cringe you can't stop replaying it in your head and it stops you from getting on with your every day life