Just spotted this today while touring Maldraxxus:

Tahonta has a white skull on the minimap, but he has a drop I need and can use (the mount). Traditionally, this would use the purple skull icon

The purple skull next to him is Smorgas the Feaster. He's correctly purple. Tooltip for comparison:

The Tahonta mount will only drop for Necrolords, which is why it has the Necrolord icon next to it. The missing mount will only cause the rare to appear as a blue skull for players of the Necrolord covenant. The Valfir, Harika and Hopecrusher rares also behave this way with their covenant-specific mount drops.

In that case, shouldn't the mount appear as unobtainable, similar to the offhand in this pic?

And just to complete the icon possibilities, to show that white skulls sometimes are correct:

Are you a Necrolord?

No, I'm Kyrian.

Thats a good suggestion! I'll consider making that change to make it more clear. For now I'm going to close this ticket since the rare icon appears to be working as intended.