Anyone that full on expects the developer to get everything into the game that they say they would like is being unreasonable.
Unless someone can point me to a release in which they said that they were 100% going to have horses in no matter what, this isn't a broken promise.
Furthermore, it reeks of ignorance towards the entire development process. It's a double edged sword too, because we as fans are stupid, and demand information from game designers (often we cite stupid BS explanations like they "owe it" to us, because we bought their previous game or whatever messed up rationalization they can think of). They wanted to have mounts in. They can't do it the way they wanted for whatever reason. Deal with it. It's not like they suddenly released the game with everyone expecting horses in it. They did their job as a developer....they let people know that a feature they wanted to get in will not actually be making the cut. If this is a deal breaker for you, then so be it. But don't get all upset because "they said they were going to but now they are not" because ultimately, it has no real effect on you. All it means is that a game, of which you have presumably not paid money for, won't have mounts. If someone is that upset about it, then he should not buy the game. It's that simple.