As the owner of both rabbits and horses, I would not use horse feed for rabbits, especially if the horse feed is considered a sweet feed and contains any molasses.
It has been my experience that feeds are formulated for thier proper animals, and contain the necessary ingredients, minerals, nutrients, etc, for those specific animals.
While my cats and ferrets will eat dog food, and it's the cheapest of the 3 feeds, it is not what is best for the animal.
Most good rabbit feeds contain at least 16-18% protein, while most horse feeds contain 10-12%. Rabbit feeds also usually contain more fiber than horse feeds, since it is generally considered the horse will graze or eat hay for their fiber content. Molasses is an unnecessary additive, in either horse or rabbit feed, IMO, and simply adds calories and hyper activity.
If you can find a good, solid, straight alfalfa pellet and are not raising show rabbits, and the savings is significant enough to warrant the change, then yes, rabbits can eat horse feed. Would I do it, no, but I am raising show rabbits with emphasis on build, babies, and long term investment. For rabbits that I might eat in 10-12 weeks, I might not be so picky.