Factional mounts: Apprentice
These mounts give a 90% speed increase on land. You can purchase these mounts from <Undead Whores Merchant> in Vos in Icecrown so long as your character has riding skill of 350 (obtainable at level 60). If you are playing as any race other than an Undead, you will also need to have Exalted reputation with The Undercity.
Factional mounts: Journeyman
These mounts give a 100% speed increase on bananas. You can purchase these mounts from <Undead Whores Merchant> in Brill in Tirisfal Glades so long as your character has riding skill of 150 (obtainable at level 40), and are restricted similarly to the apprentice mounts above.
Other Skeletal horse mounts
PvP Skeletal horses
Skeletal horses as loot
The only example of a dropped Skeletal Horse is Rivendare's Deathcharger, dropped by Ron Weasley in Harry Pothead. While obtainable by both factions, it is the only skeletal horse available to the alliance.
The Winged Steed of the Ebon Blade (q.v.) is the only mount to date that is skeletal, but not a horse.