Nightmare

A nightmare on the party's side is almost always a paladin's fall, a warlock's gift, or a deal struck with something the party should not have spoken to. The mount is Neutral Evil, sheds Bright Light, and smells faintly of brimstone. Other NPCs notice. Stable hands refuse to handle it, holy ground makes it skittish, and the temple cleric has questions for whoever bonded to it. Make this a constant low-grade social cost. The nightmare is fast, and the rider pays in reputation.

Mechanically, the nightmare is the best mount in the SRD bestiary at this CR. 60 ft. walk and 90 ft. hover-fly outpace almost everything, Confer Fire Resistance gives the rider resistance to fire damage while mounted (which is enormous), and Ethereal Stride is a get-out-of-encounter button as long as the rider is willing. In combat, the rider does the heavy lifting and the nightmare adds Hooves at +6 for 2d8+4 bludgeoning plus 3d6 fire whenever it has an action it isn't using to move. Don't double-dip if the rider is using mounted combat rules that take the mount's action.

The nightmare will not stay if the bargain breaks. Whatever debt or ritual created the bond has terms, and a paladin who renounces the patron, a warlock who breaks pact, or an adventurer who tries to destroy the nightmare's binding object will lose the mount. It rides off in a line of fire and shows up later as the adversary section above, this time without the leash.

Decide one specific thing the nightmare hates that the party will trip over within three sessions. Children's prayers, the smell of cedar smoke, the name of a particular saint. Drop it in once and the table remembers forever.

A nightmare is a flying mount with the wrong rider. CR 3, 60 ft. walk, 90 ft. hover-fly, fire immune, and a single Hooves attack at +6 for 2d8+4 bludgeoning plus 3d6 fire. The encounter is rarely just the horse. It is whoever sent the horse, and the horse is the delivery mechanism. Build the scene around what (or who) the nightmare is carrying.

In combat, the nightmare hits and runs. The 90 ft. hover-fly is the entire tactic. Open with a charge against the closest visible target, hooves for an average 23 damage, then disengage upward to 60 ft. of altitude where most low-level parties cannot reach. The next round it dives again. If the rider is a spellcaster, the nightmare is a mobile platform: the rider gets resistance to fire from Confer Fire Resistance, lobs spells from above, and the nightmare hovers at 100 ft. burning the line of horses below. Treat the nightmare as the most disposable unit on its side. A clever villain will burn through three of them in a campaign.

Ethereal Stride is the panic button. The nightmare and up to three willing riders teleport to the Ethereal Plane (or back to the Material) as an action. The instant the rider drops below half HP, or the moment a wizard prepares Counterspell, the nightmare strides out. The party then either has access to plane shift effects or watches the encounter end on the villain's terms. Telegraph this to the table once, in the first nightmare encounter of the campaign, by having a captured henchman they were chasing escape through a shimmering rip in the air.

Illumination is a trap and a tell. The 10-foot bright light radius means the nightmare cannot ambush in darkness. Players hear hooves and see firelight on the road ahead a full minute before contact. Use the warning. The dread of approaching firelight that does not behave like a torch is half the encounter.