The defining characteristic of the Starks' loyal servant Hodor is that he can say only that single word, "hodor." But a real 19th-century patient who could utter just one syllable over and over gave us great insight into the workings of our brains.
There are lots of animals, including dogs and apes, that can communicate in something we might understand as sentences. But only one non-human species has complex enough communication that they actually need grammatical rules. Say hello to the Bengal finch.