These inaccurate stories are widely believed to be true. The evidence says otherwise.
There has not been a sudden crash or disappearance of the cinnabar moth. There has been a decline in the moth's numbers like most British moths but no crash or disappearance.

From Hansard the official record of parliament:-

Shona McIsaac: I am glad that the hon. Gentleman has clarified that point. Although all hon. Members have constituents who are worried about the welfare of their horses and other animals, we also have constituents who are very keen on butterflies and moths, and we would probably upset them if there were any suggestion of their extinction.

Ragwort is important to biodiversity in this country. It is vital to the survival of the cinnabar moth. So we begin to discover how the plague started in the late 1980s. In fact, 1988 was a particularly good year for the cinnabar moth. What happened was that the cinnabar moth caterpillar munched far too much ragwort. Although that sounds like a good thing, the following year, there was nothing for the caterpillars to eat, the moths did not lay their eggs and the predator population of cinnabar moth caterpillars plummeted, so we began to see an increase in common ragwort in the late 1980s.

Website of Jonathan Djanogly MP

Whatever the numbers with ragwort poisoning, what is clear is that the plague of ragwort, and its rapid growth since the strange disappearance of the Cinnabar moth ten years ago, means death to horses.