Stat this!

[Tyndmyr]

Aright...so, my friends came up with the "impossible" creature what needs statting. I guaranteed that the playground could come up with some rules legal atrocity to make this a thing.

An undead flying lava spider horse. Note that it must be carnivorous.

[Weezer]

...By any chance have you or your players been playing Dwarf Fortress? Because that would explain how you came up with such a monstrosity.

[Flickerdart]

Horse (SRD)
+ Winged (Savage Species)
+ Fang of Lolth 9 (Song and Silence)
+ Fire Elemental Creature (MotP)
+ Ghost (SRD)

[Randomguy]

Ghosts can already fly, so there's no need for the winged template.

You could swap the fire elemental creature for the lavaborn template, a reworked version of the acidborn template in dungeonscape.

[Dr.Epic]

Well, we know the con if it's dead. Int, and cha I'm not sure. Is it intelligent undead or what? Also, if it's lava and spider, that it should have a stinger that deals fire damage. What else...?

[JaronK]

Does it need to be ridable? I'm thinking a Bone Creature Fang of Lolth Nightmare. I'm not sure it's "lava" so much as it's simply on fire, but that should get us into the ballpark. Also there's the upgraded Nightmare in the entry for Nightmares if you need something stronger. The Fire Elemental Creature template should work too.

JaronK

[Tyndmyr]

...By any chance have you or your players been playing Dwarf Fortress? Because that would explain how you came up with such a monstrosity.

One of them does play it...he might be partially to blame for this.

It doesn't have to be reasonably ridable or intelligent or any of that...you merely need to be able to hit it with a stick.

[ClothedInVelvet]

It doesn't have to be reasonably ridable or intelligent or any of that...you merely need to be able to hit it with a stick.

You want us to make you something just so that you can hit it with a stick? That doesn't seem very appreciative.

[Tyndmyr]

Look, I know my players. At some point, this dead horse is gonna get hit with a stick.

[Qwertystop]

If the template used for Undad and Flight is Ghost, hitting it with a stick is going to be tricky.

[Feytalist]

Zombie pegasus/hippogriff? That's the undead, horse and flying parts. Stick the vermin template on it for the spider part (there is a vermin template somewhere, right? I seem to remember something like that. Maybe I'm remembering the something-bred of Lolth.). Half-fire elemental. Done.

Substitute skeleton for zombie if you feel skeletal wings = flight, cause those single actions makes zombie horses sad.

[Eldan]

Simplest version I can think of: Bone creature fire-elemental arachnoid horse.

Or, perhaps, for ridiculousness:

Ghost Half-pyroclastic dragon tauric monstrous spider/were-dire horse half-giant.