The term horse breed is used in its most specific sense: A population of selectively bred horses who share certain characteristics, usually with their pedigrees recorded in a registry. They are purebred to a certain extent.
Horse Reality focuses its research on all genetic possibilities available to purebred, registered horses. As long as there is proof for certain mutations to appear in a breed they will also be a part of the game's genetics for the breed. As such, it's possible to commonly see colours in the game that in real life would be extremely rare or even avoided by breeders. A good example is the cream colour in Shire horses. In the US there is one known family carrying the cream gene. It probably found its way into the breed before the studbook was completely closed and managed to remain hidden as smoky black until recently.
All colour genes that are available for breeds have been checked and confirmed by a researcher. Even if certain mutations are rare and/or frowned upon by real life registries they still are (or will be) part of the game's gene pool. You can find an overview of all in-game and in-progress horse breeds (including their colour genetics) here: Horse Breeds - Artwork & Genetics
Hundreds of horse breeds exist today in real life. Our artists do their best to bring more and more of them to Horse Reality! This is a list of all the breeds that are currently in-game:
We keep track of the breeds in the works on this page: Horse Breeds - Artwork & Genetics. Furthermore, we have this topic on our forum, which is the place to suggest your favourite breed that you would like us to add in the future.
A feature to pair two horses of different breeds has been suggested to us many times by players. Sadly this is something we will likely never add. It would require an extreme amount of artwork to implement this realistically, along with a heavy reworking of the game’s code. Some problems could arise from it, too, such as people quickly boosting traits via crossbreeding and then breeding back so that newer generations count as purebred again and outperform the original lines. Due to this, pairing two horses of different breeds will likely never be possible in Horse Reality. One day in the far, very far future we might add “official” crossbreeds with registries in real life — but this is our lowest priority right now.