Someone more qualified will probably give you the details but bascally I
think it inhibits oxygenation of the blood. A interesting point is that
cynanide is present in a lot of stuff including apple seeds. So given
sufficient apples (say fallen apples under a tree after a very windy
day) it is possible for a horse to poison itself, if it doesn't colic or
founder first.
-- Nicco Murphy, San Diego County