TL;DR: Was dinking around in Shadowlands and learned that this mount may have been dummied out of the game by mistake. It’s also Shadowlands, so I’m probably the only one who cares at this point. 
I was buying some stuff from the Necrolords vendor in Shadowlands and one of the mounts I bought, the Armored Bonehoof Tauralus, pinged my Rarity addon when I learned it. I thought that was weird since it’s a vendor mount, so I went looking on Wowhead and saw that it’s the same mount that drops from Sabriel the Bonecleaver.
The comments on the Tauralus’ page also mention another mount dropping from Sabriel, the Bonecleaver’s Skullboar, but that was seemingly only during beta. Considering how uncommon it is for a rep vendor mount to also be a drop somewhere (usually it’s either/or, outside of holiday items), along with the facts that it dropped during SL’s beta and both the rare and mount have ‘bonecleaver’ in the name, I’m thinking some sort of mixup happened here.
I dunno what would’ve caused this mixup to happen, or even if it is one, but doesn’t it seem odd that the rare’s dropping a vendor mount instead of one that sorta shares its name?