Delugen

Imagine being so engrossed by a dream that it feels real; some might even call it a delusion.

This ottuk is one of six transparent, green "Emerald" mounts that are purchasable from Elianna in the Emerald Dream. To obtain this mount, you must:

Reach renown rank 5 with the Dream Wardens.
Speak to Elianna in the Central Encampment and complete the quest Emerald Reawakening. This unlocks an account-wide ability to obtain Dream Infusions.
You can acquire Dream Infusions by accumulating Dream Energy through various activities around the Dream, until you fill a progress bar. Speak to Elianna to check your Dream Energy progress bar.
Once your Dream Energy progress bar has reached 100%, you can obtain a Dream Infusion from Elianna, which you can use to purchase the green mounts and pets.
However! There's one extra restriction: you must already own a mount that has the same model as the green one you're purchasing, just in a different colour. See the "Other Mounts Using the Same Model" section on this page for a list of mounts that should work (they're currently untested).