Answer
Dave:
Too much onion can be fatal and poisonous to a dog. Just as chocolate is fatal to a dog.
If your dog ate an onion ring, I would NOT worry about it. A whole onion or two, I'd be more concerned.
Dogs develop hemolytic anemia if they eat enough onions. I don't think that it matters too much whether the onions are cooked or not.
The quantity of onions required is high enough that dogs can generally tolerate small doses of onions without any problem and moderate amounts of onion without clinically apparent disease, even though there may be measurable changes on lab test results.
Cats are probably a little more sensitive to onion toxicity than dogs are. I can't find an exact quantity of onions required to cause toxicity problems in dogs, but there are several case reports of onion toxicity and they involve whole onions or sizable portions of chopped onions (like a cup or more).
I think that feeding dogs meat that has been cooked with onions is pretty safe but you might want to avoid giving them the broth from around something like pot-roast if there were a lot of onions used in the cooking, just to be safe.
WinterChild:
Onions, as well as garlic, are not good for dogs. Keep a close eye on
the dog. If he/she ate a good amount you might want to call the vet and
ask about it.
Ceegt:
A lot of dogs are allergic to onions and it can be fatal to them.
Greyman:
Nothing, other than it having bad breath. My sisters dog used to eat them, she liked them for some weird reason.
