There is a business that operates in a ranch on the north side of Aertael, outside the walls, where you can buy (and sell) mounts of various types - creatures such as griffons, hippogriffs, and even sometimes dragons. They also buy and sell unusual creatures that are not mounts, but not very many of these.
Prices will vary with type of creature, training, and so on. Once per month, a new listing will become available, with a small number of creatures each month.
The proprietor is a human woman named Mikkael. Working on staff is a middle aged female human mage named Arlegast. She knows a number of ways to magically investigate creatures to be sold or bought. Also on staff is a male human animal expert named Orushan. He is about 60 years old.