I can't say for sure, but I don't think L3xisPlex meant that this sentence is ungrammatical or a bad translation. In some languages, the word for tail also functions as a common euphemism for the male genital. Perhaps that's where they're coming from, claiming it's wrong. Not language-wise, but that it is a questionable thing to teach in a language course. I feel like it's ambiguous enough. Perhaps Duo is referring to his splendid tail feathers. Silly anecdote warning. I've just recently figured out that my dog's bald spots are from hitting himself when wagging his tail. What a good boy?! We thought he had a skin problem. Wouldn't have been a problem if he could just say the words "I have a long tail." in our language.
Still, if you are going to claim that a sentence is wrong, you should provide some indication of why you think so. And in this case, the claim is not just that it's wrong, but that it's too wrong, as though it's OK to be a bit wrong, but not wrong beyond some point. It doesn't make sense, and there's no point making a statement like this if no one can figure out what you mean.
(I do like the story about your dog. What a sweetie! My guess: a lab or other short-coated retriever.)
I was also thinking of some kind of costume. Where I've lived, tail never referred to male genitalia It was something a man might say if he was trying to "score" -- he was going out to get some tail. And I don't think it meant that he was -- how do I say this politely? --that he preferred a rear position. It just meant ❤❤❤❤❤❤❤❤❤❤❤ with a woman . Come to think of it, the same meaning comes with the phrase "getting a piece of -ss" I wonder if there are more euphemisms for sex or for getting drunk?
I have such a hard time making out the consonants for "type what you hear" (snail speed would be SO helpful). The upside of this is that my guesses, while completely wrong, are often entertaining. For example, I tried "יש לי בגנר ארור", which was translated as "I have a bloody bugger". Who knew??