I've read where human rulers in ancient days built up resistances to poisons by actually taking in small amounts. Make's me wonder if in the past, dogs and cats did the same as so many of them were fed family scraps, foods that often were spiced with onion, garlic, etc.
Our family did the same thing with dogs, they got scraps as probably 50% of their diet back in the early 1960's into the mid-1970s...when my dad finally made enough money that we started buying more dog food and stopped giving the dogs everything off the plates. None of our dogs died early or mysteriously and lived into old age.
I also had a year old pup that grabbed a baby onion I dropped....the stove was at the edge of a wall and the living room transition area started there and that was where my 2 dogs, Sampson and Hey You were laying down, ...the onion rolled right to the dog and she grabbed it and ate it before I could do anything. At that time I didn't know onions were bad for dogs, but I couldn't believe she ate it, even when upon first biting down on it, she realized it tasted bad. If Sampson had grabbed it, she would have spit it out upon first taste.
Stormy