In high doses capsacin can be an irritant and cause temporary gastrointestinal issues in dogs AND humans It is also a neurotoxin (GLaDLY only short term effects) The dosage required for children and dogs may be lower as they are smaller source It has not been found to be lethal even at high doses in dogs and is quickly eliminated from there system. Source: 1, 2 Still don't put your dog in pain (they can't tell you when it hurts) and the side effects of high dosage can be dangerous just like in humans.

Guinee pigs however are very sensitive they can have long term health effects and it can be lethal so dont feed your guinee pig chilli (capsicum is great tho). Also other small mammals like rats, mice and rabbits can easily have a lethal dose because they are so small.

But yea from my research it is no more toxic for dogs than it is to humans