On Oct 24, 2020 we've finished the HatMouse House on stream! The build includes some non-canon storylines that emerged during the stream talking about HatMouse. Where did they come from? Why is HatMouse so large, but a tiny mouse hole is on the front of the house? Why does he sell hats? (That one we never resolved.) But I think what was created is something beautiful.
HatMouse's origin story, according to Rootabega, is that Joja Co used to store their by-product waste from the Joja Blu at this house, which was once one of their houses where their scientists lived. The house was abandoned after a purple and pink meteor busted through the roof and Joja deemed it too costly to fix and just left it. During the night of the meteor strike, one of the barrels of waste was knocked over and mutated a mouse. Thus is the humble beginnings of HatMouse.