I have a 15year old horse, who grazes for a few hours each day but mainly has haylage and a small bit of hard feed. I bought some linseed the other day by accident, thinking it was something else, but when I realised I thought I'd check out what it was used for, and the benefits seem to be fitting for my horse seeing as she mainly has haylage for forage. BUT then got all worried after reading all these dangers of the acid hurting her if it wasn't prepared right and now I'm not sure whether to give it to her or not....
It isn't in the form of oil, it is what i THINK is referred to as a 'linseed cake' although not completely sure of that, it is solid and about the size of a 2penny piece, but rectangular, and looks almost like a dark brown biscuit.
Is it safe to feed her this type of linseed without having to prepare it in any way before hand or do i need to soak and boil it as you do with the oil?

It does sound like linseed cake.
If your horse is on mostly haylage and a little hard feed I would not give this to her to be honest. It is high in very energy so not ideal I suspect. Mostly given to cattle in that form these days I am told (rather than horses) - but that is heresay.

Why not take it back and see if you can swap it for something else - or sell it on. Try dropping into a big yard where they have competition horses. (take a few samples to people in clear zip freezer bags)

Linseed before treated - as a seed crop is a small brown shiny seed.It needs to be boiled for several hours - a minimum of 4 is best - although you could use a microwave but not sure how long you would cook it in that. This is so that the seeds split and the prussic acid is released.

The prusssic assic is the poison you were referring to and yes it IS dangerous if fed before this acid has been released. YOu can then (after boiling the seed and straining it and throwing the seed away) feed the liquid it was cooked in - called Tea. Or the Jelly which is left.

It is normally only fed to high performace horses and to animals which are in need of high energy. It does of course provide a shine to the coat as do most oil based / type foods - but that is a side effect and not a reason to feed it.

I would just accept the fact that you spent a little dosh on something you dont need and sell it on if you can.
Good luck

Packhorse