My first two horses bred were level 19 grey tier ones. I based that off of the horse breeding calculator found here. Including my first tame Spooky.
It’s not much but it is a start.
If you are interested in breeding at all take a look at the breeding calculator and Brank’s videos on horse taming. He’s a positive guy and has some good ideas. The videos are great if you need to get started. Dulfy also has a great resource on getting into taming and breeding if you need something with less video.
As of the date this post was written these links are accurate. I can’t guarantee that they will still be active at later dates, but for the moment they’re good.