I thought that the cat would probably have eaten the bits that came out when the horse was opened up...intestines, liver, stomach, heart, kidneys, that sort of thing. I assumed that the rest of it would then have been taken by the knacker's man.
As for the difference between an attic and a garret, I'm not sure. You hear of poets starving in garrets in Paris, but a house (especially a biggish house) often had several attics. Maybe a garret is at the top of a small tower? Or perhaps there was a front attic, a back attic, and then another floor above that, the garret (what would nowadays be called the penthouse!).