How about a mix of them all? Halflings would probably just use whtever farmyard animal was to hand as a mount I'd have thought...
I have thought about that, but it would detract from the unit coherency and I'd prefer to spend a bit more money on more rider variants than on half a dozen different mounts.
If I had to pick one though, Roosters every time!
Yes, it's silly and yet it fits so very well with the fluff. And I've been to a chicken expo once and was amazed by the size of some of those beasts, it wouldn't be such a stretch to have a breed that's selected on size to arrive at something big enough for a halfling to ride.
Would also open up some interesting painting opportunities.
Cockatrices.
I always thought those basically were fantasy chickens. Never seen a representation of one that really appealed to me though. Though the rules could perhaps be adapted to use with roosters.