One of the lists I was looking at is here.
http://en.wikipedia.org/wiki/List_of_plants_poisonous_to_equines
Some of them do list caveats like not using in hay, however when looking at sites offering cover crop seeds for sale today, a couple of them noted not to use red clover or sweet clover anywhere that is planned for use for horse pasture. There were similar notes for mustard and sorghum.
I distinctly remember sweet clover being a favourite of the horses I was around when I was young, though...
The problem is that just by reading websites, I have no way to confirm or deny which list is accurate and which list is not. That's why I'd like some advice from people who actually own horses and have experience with these plants
It is frustrating because there is such a tendency for people to start yelling "I'M SUING!" every time something goes wrong that people are now posting warnings on EVERYTHING, even if the chances of anything going wrong are slight. I mean if you buy a fake firelog from the store now, it has a "warning, flammable" label on it.... *eyeroll*