Players keep asking "What can a Thestral breed with?" or "Can an arabian and a pegasus breed?"
About 1-2 years ago a tab was added in your stable that tells you that.
The tab is called "Check breed" and it works in 2 ways:

1. Select the desired breed on the first box, and leave the second box as Any. It will show you all the breeds that your desired breed can breed with.
   Example: If on the first box you select arabian, when you click the check button, you will get the following result:
   Arabian can breed with mares of the following breeds:
   American Quarter Horse, American Saddlebred, Anglo Arab, Appaloosa, Egyptian Arabian, Fell Pony, Morgan Horse, Polish Arabian, Spanish Arabian, Thoroughbred, Welsh Pony, Arabian, Donkey, Rainbow Zebra
   
2. The second form means you select on both boxes a breed and the game will tell you the result.
   Example:
   First box: Arabian
   Second box: Welsh Pony
   Clicking the check button will give you the following answer: The resulting breed is Welara!