interesting creature to true polymorph a galvan magen into, duplicate the 7th level create magen spell through the 9th level wish spell while concentrating on the 4th level aura of life spell (eberron races or ravnica background or simulacrum of an ally or the ally themselves) or being inured to undeath from 10th level wizard school of necromancy so the maximum hit points aren't reduced; one new nightmare every two long rests; true polymorph another magen into dryad for up to 30 daily goodberries and access at will create or destroy water spell by true polymorphing either self or simulacrum into kraken priest perhaps dismiss duration before the end of the hour so the form reverts after casting enough water for everyone; by the way true polymorph another magen into a berbalang to translate every language perhaps true polymorph poisonous snake familiar into a twig blight or a young kruthik to expand special sense while avoiding mechanus primus network by not turning them into a monodrone; twig blight's blindsight with action provided by the find familiar spell might be the play when you want to try counterspelling a potentially suspended/submerged dimension door or similar spell through a sleet storm/darkness spell or similar effect we are groot

Does anyone else think of the horses from Krull every time they see these?

Could you turn a (willing) nightmare back into a pegasus with the Atonement option in the ceremony spell or would it just be a Chaotic Good Nightmare? I'm asking logically, not necessarily mechanically.

if you have tamed this guy, can you summon them from the ethereal

Quote from Lucklord1 >>

So is it possible to have one as a mount? That would be cool.

 If you use find steed to summon a horse in Avernus, the horse is always a Nightmare instead.

my character has one as a mount, but not through a spell -- we came across one and instead of killing it we helped it out and now he's my bud 😆 

Same thing here

This would be amazing for a Hell campaign. Although it could be pretty OP for it too

Certain playable races use them in the lore. Also they can be found naturally in Certain planes. You have to completely dominate its will (they have strong will's and personalities) to ride it and if you fail it'll eat you. It can choose to disobey you and they will get very aggressive if you don't feed them a human every month

My DM just let me have one as a mount after encountering it in the Abyss, and my Druid has never been happier :D

You can mount them if your DM allows you to, and most likely they'll have you roll animal handling (Prob 15 DC possibly 20 DC) If you have MotM you can get infernal tack, which is a set of horse accessories (Saddle, bridle, reins etc.) that is used specifically for the Nightmare. The description says that a Nightmare with the tack is bound to serve whoever wears the spurs in the set until either the wearer dies or the tack or spurs are removed. You can also use  https://www.dndbeyond.com/magic-items/figurine-of-wondrous-power-obsidian-steed one of these, or find greater steed. Remember though, that it's always up to your DM.

Quote from Schork >>

The best mount

 Yesssss

why does it walk on air faster than it walks on land? why would it ever walk on land again? just pretend but fly at 1.5x speed

I love the lore of these being tortured wingless pegasi

Quote from scarberian >>

Yes.  In the new module Descent into Avernus there is an opportunity to do so.  

 Hi

Yes depending on your DM/GM. I personally allow it, but require that the user have the "infernal Tack" which are caried by narzugon. 

Of course you can. They are mounts for powerful bad guys. My Paladin of Asmodeus rides one. ;) 

Quote from WargdoTheAnimeMan420 >>

HOW??!?

 It uses its hatred. If someone tore off my wings, you can bet I would learn how to fly without them just out of spite

The figurine of wonderous power is a magical item that gives it to you as a mount, but let's you take it around as a figurine when not using it