You can have an Obsidian Steed, which is a Very Rare Figurine of Wondrous Power. It can turn into a small statue that fits inside your pocket, understand your language, and is friendly towards you and your companions. And it can turn into a Nightmare, which you could use as your steed. Yes, it would be really cool. I wonder if you'd get burnt riding it though.

Quote from ElephanteTheSmall >>

You can have an Obsidian Steed, which is a Very Rare Figurine of Wondrous Power. It can turn into a small statue that fits inside your pocket, understand your language, and is friendly towards you and your companions. And it can turn into a Nightmare, which you could use as your steed. Yes, it would be really cool. I wonder if you'd get burnt riding it though.

 Confer Fire Resistance. The nightmare can grant resistance to fire damage to anyone riding it.

The Find Steed spell in Desent into Avernus will give you a Nightmare

This is the perfect mount for an Oathbreaker Paladin!!!  My DM says that I can summon this using Find Greater Steed with my Oathbreaker when I get the spell.  I’m so excited!!!

there is the obsidian steed figurine of wonderous power and there are the spurs that narzugons wear

Hey DMs,

Give your Oath of Redemption Paladin players the chance to redeem a nightmare. That story writes itself, my dudes.

This is such a cool mount I might give this to one of my players.

Quote from JohnSparta >>

Hey DMs,

Give your Oath of Redemption Paladin players the chance to redeem a nightmare. That story writes itself, my dudes.

 Love that. But don't give it wings back (tough that would be cool) make it go white with blue holy fire for hooves, mane and tail. 

What this mothaf**ka fast like

Love them so much

So how does the Ethereal Stride work? I see the description from Etherealness spell (which is great for understanding ethereal mechanics) but after spell ends you return to the same place you started. Is that how the Nightmare's Stride works too? Still cool but actual movement would be great too.

(My CG Fighter sold his soul in Avernus for +3 weapon and turned LE, which allowed him to use the Infernal Tack we looted later, and now has a Nightmare. He is now back in Balder's Gate.)

Quote from WargdoTheAnimeMan420 >>

Why does it have flying speed?

 it used to be a Pegasus so now it can walk on air, it is a magical being. don't think about it too much

yes if you use the figurine of wondrous power( obsidian steed)

it summons one but you cant be a good alignment

 Aren't Nightmares also meant to be able to understand abyssal?

100% using this as a mount for the headless horse man

I gather that this creature has changed from the list of comments. Originally this was a creature that "may" allow one to ride it, but it is considered an intelligent one so has its own ways and will not likely take orders like a pet or tamed thing and as well it would only allow one of evil to ride it. My original aim in mine was that it was the polar opposite of the Unicorn, thus being able to taunt one in their dreams and even cause them to have mental issues prior meeting. Once they have met this creature it could then taunt them through the dreams at any time they were sleeping. 

Despite being wild and roaming creatures, they were known to serve willingly as mounts during missions of malice, but only for those of sufficient wickedness and ability. Those that wished to ride them would be wise to remember that nightmares were not simple steeds but fiercely independent and untameable fiends with their own instincts and agendas. Prideful and egotistical, nightmares often acted upon their own wills even if such actions ran counter to the goals of their supposed masters and sometimes even at their expense. Even if both parties desired to commit evil deeds, their exact methodologies could be incompatible and attempts to force them into servitude often ended poorly. Only after forging a bond with them over time would a nightmare become loyal to a rider and only then would the two become true partners in villainy.

https://forgottenrealms.fandom.com/wiki/Nightmare_(creature)

I play a chaotic evil ixlan vampire(plane shift) were raven basically I accidentally activated an artifact back in the mtg world that transported me to the mists between the domains of dread and then took refuge with some were-ravens after a while they allowed me to take 1 magic item and my dm gave me an infernal tack  so I now have a pet nightmare named Jeffry and I'm going to kill Strahd by having Jeffry kick him

How do I tame one of these?

https://www.dndbeyond.com/magic-items/4980837-infernal-tack