I’ve never thought about Cowslip’s family but I like to think they are like 100 times less creepy than he is
They probably are

Does this ring a bell to anyone? I can't remember where it was from.
I can't remember hearing that before, I'm afraid. I quickly checked the 2016 radio drama, and that's not where it comes from. However, there have been several other radio adaptations, that I haven't listened to. Perhaps it comes from one of those?
I remember that, in the book, it was said that the warren existed long before the farmer started feeding and snaring the rabbits. However, that doesn't exclude Cowslip, and maybe others, having started their life/lives as human companions
