Silveras said:
A reasonable ruling, but because it is not stated anywhere, it would also be reasonable to say you can't do it at all. That's why I said it is in house-rules territory.
SRD:
Create Undead
Necromancy [Evil]
Level: Clr 6, Death 6, Evil 6, Sor/Wiz 6
Components: V, S, M
Casting Time: 1 hour
Range: Close (25 ft. + 5 ft./2 levels)
Target: One corpse
Duration: Instantaneous
Saving Throw: None
Spell Resistance: No
A much more potent spell than animate dead, this evil spell allows you to create more powerful sorts of undead: ghouls, ghasts, mummies, and mohrgs. The type or types of undead you can create is based on your caster level, as shown on the table below.
Caster Level Undead Created
11th or lower Ghoul
12th–14th Ghast
15th–17th Mummy
18th or higher Mohrg
You may create less powerful undead than your level would allow if you choose. Created undead are not automatically under the control of their animator. If you are capable of commanding undead, you may attempt to command the undead creature as it forms.
This spell must be cast at night.
Material Component: A clay pot filled with grave dirt and another filled with brackish water. The spell must be cast on a dead body. You must place a black onyx gem worth at least 50 gp per HD of the undead to be created into the mouth or eye socket of each corpse. The magic of the spell turns these gems into worthless shells.
It specifically says, 'one corpse' and 'a dead body'. Nothing at all about what sort of creature it had to be beforehand.
Animate dead has the exact same wording, it makes skeletons or zombies based on the creature (as listed in the monstrous manual). Since there is only one listing for those in this spell then they all come out more or less the same. Physical appearences would differ according to the dm, but the stats would be exactly the same.
Anything else is a houserule, the rules here are not ambiguous.