Dogs and Cats are the two known animals that can not eat chocolate and I'm sure there's other animals out there that cannot eat chocolate as well but it's not really known.
Mice and Rats however can also eat chocolate without a problem so Humans aren't the only ones that can eat Chocolate.
Theobromine is a bitter alkaloid of the cocoa plant found in Chocolate which is what is poisonous to most animals including dogs and cats.