I just rewatched with my wife and 3 kids (13, 11, 9) and my wife and 13 year old both said "what???" when Spot devolved into a lizard (my same reaction 20 years ago). Afterwards, everyone basically liked it but thought the science was really dodgy. Entertaining episode though.
I don't remember exactly what Spot devolved into, but as all mammals have reptilian ancestors, that makes some modicum of sense. But how does Barclay "devolve" into a spider?