Ghostwolf Pup were an accidental and rare Direwolf drop at the launch of the game (July 17th 2017) and were promptly removed with Launch Patch 1 on August 1st 2017. They could be grown into Tame Ghostwolf, but you couldn't do anything with them - as they weren't supposed to exist in the first place.
Then the Ghostwolf Mount was released as an exclusive Referral reward for referring 25 players between November 15th and December 14th 2017. This mount was eventually converted into a skin with the release of the Percival content update on July 10 2019.
For the duration of the above mentioned Referral season, you could also exchange your (useless) Ghostwolf Pup or Tamed Ghostwolf for the Mount version.
Elsa's Reddit answer about Ghostwolves
Other Baby Animals