Please don't.
Cats don't tolerate aspirin. It can be fatal. Same for Tylenol (acetaminophen) and Advil (ibuprofen).
Dogs can die from Tylenol (acetaminophen) or Advil (ibuprofen), or suffer severe liver and kidney damage.
Dogs can have aspirin in very small doses, but if you take your dog to the vet, they can't immediately give your dog steroids or other (safer) anti-inflammatory meds. To be safest to your dog's kidneys and liver, you need a 5 day wait period between between any of these.
Cats really don’t tolerate anti-inflammatory medications very well. They need other approaches to pain management.
It's better to only give pain meds prescribed properly by your veterinarian.