It's probably just because the feebas gets noticed more and that's what people end up posting on forums as it's rare. Nobody would think to link a screenshot of a carvanha. It's just less interesting.
It doesn't seem like they actually change by season tho. They just appear differently and sawsbuck has it's spring form in winter so something isn't going right
You can try without hordes as well. Personally I've found 3 shinies specifically by hunting singles and 1 which just showed up as I made my way to some hordes.