You can buy this mount in the Affinity Rewards section of the store using Affinity Currency.
Affinity is earned by logging in each day as a patron (5 for the first log in of the day and then 1 per hour for 3 hours bring a total of 8 per day).
You can also collect Affinity from day 7 of the rewards calendar.
Affinity is region wide. (Shared by all your chars on NA or EU)
You can see how much Affinity you have by opening the currencies tab of your character sheet.
This mount was originally available during the Arclight Ascendancy Promo Week.
This mini event may be made into a Call to Action event in the future.