factual donkey
A donkey who knows the truth and will tell you if you're wrong, even if it's in the middle of a math test
My friend is a factual donkey. He told me I was wrong on the test, even though I had a pencil in my mouth.
My boss is a factual donkey. She said my report was wrong, even though I had a hangover.
My neighbor is a factual donkey. He told me my dog was a donkey, even though the dog had a hat on.