Let's get Froggy and talk about how to get your Arboreal Gulper.
Head over to Ardenweild and start killing. That's right, start killing everything in sight. I tried to do mine near the spawn point which you will need once it's dead @ 32.42 30.26.
Any of the mobs in Ardenwead have a chance to drop the unusually large mushroom. It's more likely if you take a group of people because you only need one. Drop chance is 1-5 percent.
Once you have it you have 30 min to get to 32.42 30.26 and click the Damp Loam to summon the “Humon'gozz.” Once this Humon’gozz is dead though you get a 100 percent chance at your Hippity hoppity, the Arboreal Gulper Mount to everyone that kills it.
You will need a large group of at least 5 depending on your group's skill. He's not an easy kill. The Loam is also really surrounded by kill mobs. It's important to note you cannot learn this mount till you are lvl 60 but you can receive it.