The question of why horses weren't included in Cottage Living was answered by producer Antonio Romeo. (Not sure if I can post links as a newbie, but it's posted on Twitter.) In a nutshell, the team wanted to add more gameplay for horses, and they didn't fit the "garden-to-table" theme.
Personally, I assume horses just weren't ready yet. They probably would've functioned like cows and llamas in Cottage Living, if included, and that would've been a huge mess.
I'm OK with horses having their own pack (I assumed they were going to be a GP though), but I'm expecting a lot of horse-related gameplay as that was the promise. The livestream will be enlightening.
