It may be intentional since you get both genders but I did notice a discrepancy in prices in the shop. All pairs of animals seem to be the price of two of the animal with a small discount. Example Horses cost 150 each or 275 for a pair the same holds true for all transportation animals except donkeys. Donkeys are 75 each or 275 for a pair (Same as horse pair)

To recap. A single horse is 150 and a pair is 275 so you are paying 91.6% of the cost of two individuals by buying a pair.

A single donkey is 75 and a pair is 275 so you are paying 183.3% of the cost of two individuals by buying a pair. (You could get 3 donkeys and be .67 on your way to the 4th for the same price)