If your horse eats a bunch, the tannin can cause laminitis or colic. I grew up around a lot of oak trees. Once they've been rained on and thoroughly soaked (the fallen ones) the tannin is leeched out and they aren't so bad. For some reason horses will consume them like candy, though, so you may just want to fence them away from them until they have been rained on a lot or aged some and the horses loose intrest in them.
If properly soaked, they aren't toxic, and are quite edible, even for people. But yucky if you ask me Still, anything in large enough quantities can be harmful. I don't know of them actually causing weight loss, but I never looked into that. I think I"m going to check out the link that was provided ...=)