Skeletal gryphons are undead gryphons. Gryphons can also become spectral gryphons in undeath.
This article or section includes speculation, observations or opinions possibly supported by lore or by Blizzard officials. It should not be taken as representing official lore.
Due to the only pre-WotLK example of this creature type existing by the unused Karazhan flight point (and Netherspace to some extent) with a very generic name, it could possibly be thought that their role was going to be the mounts that that flight path used.