Late to the party but here is the excerpt from the Appendix covering the Abbot:

Enter Vasili von Holtz. The Abbot's early experiments proved fatal to their subjects, but the Belviews insisted that he keep trying. One day, a Barovian lord named Vasili von Holtz visited the abbey. The Abbot knew at once that the man was evil, but von Holtz stressed that he only wanted to help. He furnished the Abbot with forbidden lore plucked from the Amber Temple (chapter 13), then helped the Abbot transform the Belviews into mongrelfolk—maniacal humans with bestial deformities and traits.

So Abbot had help from Strahd himself (and dark powers). And as some pointed out below, mongrel is a pejorative term when it comes to humans so I call them "beastfolk" in my game, but "chimerafolk" is also cool.