Considering its a paid mount, it should receive each and every major current and future mounts features that they can get. Simply to justify its price.

Currently Sleipnir is missing more than one, heres all the mounts features and those from Sleipnir:
- Unique music, yes.
- Flight, no.
- Mount action, no.
- Carry two players, no. ( But that makes sense )

Anyway, you get the point.
"Free" mounts are getting the flight treatment but not those we actually spend extra money to get ? ( Ok i have received mine as a gift but still ) That's stupid, because it would certainly boost its sales knowing that Sleipnir could be used in HW areas.