No. They won't eat Ragwort itself when it is growing, it is only when it is dried and included in hay that they eat it and are poisoned.
Ragwort is a Senecio, but Brachyglottis is probably a separate genus within Asteraceae.

They carefully go around it but its dangerous dry as Owdboggy says it loses its bitternes, the other problem is that it seeds everywhere and has to be pulled out by hand, but can be poisnous to us as well so stout gloves needed.
Its good point is that it is home to the cinnabar moth....thats all the caterpillars eat!

The plants do contain Pyrrolizidine alkaloids, which are a problem in ragworts, but so do 3% of the world's plants.

There is minimal risk to the horses as they aren't actually in contact with the plants and they have evolved to avoid them.

The whole fuss about ragwort is very much exaggerated. In fact the Advertising Standards Authority has stopped
adverts claiming large numbers of horse deaths.
For more information see these websites.
http://www.ragwortfacts.com
http://www.ragwort.org.uk
http://www.buglife.org.uk/conservation/currentprojects/Habitats+Action/Ragwort
http://www.swanseafoe.org.uk/ragwort-the-sense-and-the-nonsense.html

Thank you for your swift responses to my concern. Just to be on the safe side the plant has been moved back from the fence and the electric wire activated to stop the horse leaning over and feasting on the garden in general.

I appreciate all the helpful answers and links are very useful.