You might want to check all the scenario suggestions in RT.
The goofy fun stuff in the scenario section of the RT book usually references things that are already dealt with in the rules. The scenarios mention things like Ambulls, Cthellean Cudbears and Grox for example, but I dont think that the Death Ferrets get a mention.
It is possible that the Death Ferrets are mentioned somewhere else in the RT book, but I dont think so. I devoured that book as a boy and had a pretty encycopaedic knowledge of it for a few years (although much of that faded as I got older). I could be wrong, but I dont think that Death Ferrets get a mention.
I've got from WD 94 onwards and all the RT era stuff, I've never seen it mentioned either.
I jumped on the 40k train around the same time and I struggle to remember anything about the Ferrets. I am certain that I havent seen any rules and any background beyond a mention of the name.
it is probably the result of someone sculpting a ferret...
The Gyrinx and the Ferret look like early Jes Goodwin sculpts to me. A bit like the old Wood Elf beasts in style maybe.
...getting it cast and then they had to put it somewhere - so it ended up in the RT section, but without any rules attached to it
This is what I strongly suspect too, but I hope to be proven wrong.
Thanks for the contributions so far. Anyone else out there have any further info?