Probably the Caspian - was used by the Persians as chariot horses and believed to be extinct for a millenia until it was rediscovered in 1965 and there are only a handful in breeding programs to try to bring them back.
I'd say the Prezwalksi's Horse-There are roughly 1,500 of them left in captivity and 400 in the wild. All of them are the descendants of the same 14 founder horses.
It changes everytime howrse bring a new breed out. At the moment it is Fjord as they are relatively new. Otherwise, it is the Peruvian Paso.
There isn't a 'rarest' breed you can own. You can RENT a Pegasus or Unicorn for 1million on Cloud Isle for 2 real hours, that's as rare as you could get, but you can't own one.
The world's rarest dog breed is the New Guinea Singing Dog.
The Eriskay Pony, and the Akhal-teke Horse, are the rarest horse and pony breed in the world.
Sumatran rabbit
Havana Brown.
Ndabezinhle Phiri
there are no horses
pink =D
the rarest fish is the super sized golden trout/salmon cross breed
The most rares is derker whale
the germish spotted dear rabbit