I always thought that the reason cats and horses are extinct was to mimic the mad max world of most wildlife being extinct/hunted to extinction due to environmental collapse. To me, horses have always seemed too, I don’t know, “elegant” to fit it the fallout universe. When I think of post-apocalypse, I don’t think of everyone going back to using horses, but using ramshackle cars and bikes. Packs of rabid dogs feel post apocalyptic, but housecats don’t really fit. Maybe if they were more mangy looking than the ones in Fallout 4 I could see it. Oh, and at least one donkey is known to have survived up until the events of Fallout 1, so there’s that.