Oooh, and another one that is obscure/underused (although perhaps deservedly so).
The Hippocampus, half fish/seahorse and half horse. I have never used one in my entire DM career, and I cannot recall seeing one in any modules. Even in cases where you might expect to see it like in the Saltmarsh series. At least they do not show up in the recent update, and I do not recall them in the originals.
Of course, I likely have missed an appearance somewhere.
Not the greatest monster since the 1e version was CG and therefore not something you would likely fight, but it is a mythical beast.
Cheers