Sounds like "Counter"..my dad used it on our farm. Its a pesticide but used incorrectly will kill a dog in no time at all. I lost two pets to it when I was a kid. It killed them so fast that they died beside each other. It has been know to be mixed with peanut butter to kill mice as well, but if another animal eats the mouse it will die too as well as what ever eats that critter as well..so on and so on..
It's commonly used on corn. Counter is no joke.
Sorry to hear about your lost.........