Mostly, horses are quite sensible (in this department anyway) and won't eat ragwort whilst it is alive but when it dies back it is a risk or if it is cut and baled in hay. If horses are hungry enough they will eat it dead or alive. Also, once you've pulled it, be careful what you do with it.
When you are pulling ragwort you may well notice a little yellow and black caterpillar. This is the larvae of the Cinnabar Moth. It feeds on ragwort, making itself poisonous and therefore unattractive to predators. Pretty clever hey? Apparently, in some countries they have introduced the moth to help to control the ragwort.
So, as you rub your sore back and pull the final piece of ragwort from your paddock, spare a thought for this pretty little moth!