[blackarrow]

From what I've seen, you're pretty far ahead of yourself in saying that these horses were deliberately poisoned in some way. The vet in the Today report you posted said he thought it was very unlikely that they got some sort of tainted shot.

Horses can be very sensitive to things they ingest - it's also very possible they ate a poisonous plant in their pasture or something, completely accidentally. It's very sad that they died I think we ought to wait before deciding they were killed by the competition!