Koko the Gorilla
Koko the gorilla understands more than 1,000 American Sign Language signs and around 2,000 American English words. Naysayers argue that Koko does not understand the meaning of what she is communicating and that her expressions are used solely to elicit a reward. If that were the case, how would one explain her stringing together words she knows to describe an object she doesn't know the word for - such as 'ring', which she called a "finger bracelet" or a 'mask', which she called an "eye hat".
Koko is also one of the few non-humans to keep a pet. Her relationship with her kitten, All Ball (which she named because she felt he was round), was featured in the 1987 children's book, Koko's Kitten.