User blog comment:Solara184/Breeding projects for the month of March/@comment-38439752-20200306192144/@comment-38439752-20200306195054

Or they are rolling out a new horse model at the same time ;) Actually now that I think about it, I don't think the updated horse models had anything to do with the selection, apart from the fact that new breed t4s haven't been designed on the old models at all so they couldn't come out before this update took place.

In any case, I think we'll see the rest of the common horses updated during this month. Maybe even some of the Exclusives. Not sure if the week long break in T4s has something to do with the update schedule for the rest of the horse models.