If your timing is right, then the Arboreal Gulper in Ardenweald is one of the easiest to obtain by far. While you’ll be working on a time limit, you won’t have too much of a challenge ahead of you – if you know where to go, anyway.
If you’ve adventured around Ardenweald for long, you may have seen the Unusually Large Mushroom drop. This green-quality item has a 20-minute timer before it expires, with the flavor text implying that the mushroom will die if not placed in loamy soil before the time is up.
This mushroom can drop off of any Ardenweald creature, though some have a higher drop rate. You can even get them in Ardenweald dungeons like Mists of Tirna Scithe – which can be a bit of a hassle, given that your 20 minutes starts the moment you grab it.
Head to the northwest corner of Ardenweald – if you have a coordinates addon, 32.48/30.28 is the rough location. There, plant the mushroom in the Damp Loam.
Once this is done, the mushroom will swiftly grow into an… unusually large mushroom, fittingly enough. After a few seconds, the mushroom explodes, causing the rare mob Humon’gozz to jump out.
Humon’gozz has a considerably high amount of health, so consider bringing some help to take him down. Once you manage to kill this humongous mob, you can loot the creature to obtain the Arboreal Gulper, which so far seems to have a 100% droprate!
From there, it’s as easy as using the item in your inventory to teach you the mount and obtain it from your collections tab. It can’t fly – nothing can in the Shadowlands, but you can fly some mounts back in older zones – but it’s an aesthetically-adorable big old frog mount for you to hop around the Shadowlands on!
If you’re having difficulty getting a mushroom to drop, you’ll likely see some people advertising that they’ve earned the drop in the Ardenweald general chat. The mushroom is far from rare, which is part of why this mount is so easy to obtain.
You also may consider farming the mobs around the spawn for the Damp Loam to get a mushroom, effectively eliminating the need to travel and plant the mushroom within 20 minutes. The loam does have a spawn time, so it isn’t a guarantee that you’ll see the needed plantable loam to spawn Humon’gozz.