Dreamhoof
Sold by Elianna. Be sure to sleep with some grass under your pillow so you can give this critter a snack while you dream. Magic type. Summoning Juniper worked for me to let me buy it
Dreamhoof is a pet vendor item in the Emerald Dream purchased from Elianna using Dream Infusion currency, requiring you to summon a Fawn-type critter to unlock.
Guide awaiting review
Summon a Fawn critter
Summon one of these Fawn-type critters in front of Elianna: 
Ivy, 
Mitzy, 
Spruce, or 
Juniper. Spruce and Juniper are sold by the quartermaster next to Elianna.
Purchase from Elianna
Find Elianna at 50.2, 61.8 in Emerald Dream and purchase Dreamhoof using Dream Infusion currency, which unlocks account-wide at Renown 5.
Tips
You receive one free Dreamhoof on your first character to reach Renown 18, and can collect Dream Infusion gradually from Paragon chests, weekly event satchels, dreamseeds, Superbloom events, treasures, rare kills, and creature kills in the Emerald Dream.
