I had a dog get into a Valentine's heart shaped box of chocolates I gave my wife.
She had left it on the coffee table overnight.
The dog was 12 at the time and lived 5 more years after this incident, but she got really sick.
Touch and go for a few days.
That same dog got into some neglected Sonic tater tots with cheese, and ate the cardboard where some of the cheese had melted on it.
That dog looked like a coyote mix, and I wouldn't be shocked if it was.
Onions are another thing to be careful with.
My 2 hound dogs (a beagle, and a foxhound), love 'em, and every time I bring a What-a-burger home, I've got to get to any I drop before they do.
Onions can do something weird to their blood, but I don't think mine have been harmed.
Not sure the amount needed to cause damage, or whether they are cooked or not, if those things come into play.