Saharaa, it's possible that your TB mare will still be "pony sized" as a fully grown adult. I don't know if that's supposed to happen, but sometimes the game goes wonky. That's how I ended up with Tiny Wubs.
So, I created a second account to have a dedicated breeding account as this is kinda just a junk drawer. I had to leave before choosing starter horses, and now I don't have the option to. Where on that account can I create them?