And you keep failing to understand that right hand man is cannonically different from first mate in this manga. If it wasn't the case, the likes of Marco and Zoro, explicitly addressed as their captain's right hand men, would have been given those titles instead of doctor and combatant in their vivre cards.
That's what the word means and how it has been used in One Piece, I don't know what you're talking about.
If you want to stick to One Piece logic I recommend you to check the vivre cards and see which right hand men aren't given the title of first mates. Spoiler: Zoro is one of them. How some right hand men are also given the title of first mate is probably a matter of hierarchy and is more related to how each captain views their crew (every Whitebeard subordinate is his son, Luffy doesn't see anybody above any other as every crewmate is equally important), and therefore it is significant to make the distinction because it could have an important impact on the dynamics of each group. But I guess it is easy to simplify things to "right hand man = first mate" while ignoring why the official material took the trouble to make explicit distinctions between those positions.
Which isn't first mate (副船長).
Tell that to the vivre cards that gave him the role of COMBATANT and Marco the role of DOCTOR even though they're both their captain's RIGHT HAND MEN. I'm just sharing canon information, if you don't like it go and discuss it with the people responsible for this manga.
And hippopotamus are horses who live in rivers because that's what the two words literally mean. Go learn actual English or at least open a dictionary, please.
Urouge didn't pronounce that word (check the Japanese panel) and Bartolomeo added a "(by me)" that pretty much subjectivizes his statement to the maximum degree.
