For this, I'd fall back on 'backwards compatibility' and dig up the old ghoul, ghast, wight, shadow, etc. templates from various 3E books (the shadow creature template, IIRC, was also available in Dragon magazine).
Libris Mortis has templates for Ghost Brute, Gravetouched Ghoul, Mummified Creature and Umbral Creature, which can be pilfered and applied to pretty much anything you want to make into a Ghost, Ghoul, Mummy or Shadow by whimsically ignoring whatever creatures they advise applying the templates to (for instance, Gravetouched Ghoul is mostly meant for humanoids, giants, monstrous humanoids, etc. but there's nothing in the template that goes boom if applied to a dragon or outsider or dinosaur).
A non-template method is also usable from Libris Mortis. Take your Bulette and give it 8 levels of the 'ghoul/ghast' Monster Class, and it's a Ghast Bulette, ready to rock and roll. Monster classes for Ghoul/Ghast, Mohrg, Mummy, Vampire Spawn and Wight are available, making Libris Mortis even more of a 'go-to' book for this sort of thing.
Excellent. One of my players has the Libris Mortis, so I think I'll borrow it and see what I can churn out with templates and class levels.
I ran into this problem when I was creating a nest of wights in Katapesh. I thought, "What creatures would be most likely to have been attacked by the wights and converted?" So there were some humans, some gnolls, some goblins, and a pugwampi wight. It just didn't seem right to give the tiny gremlin the same stats as the gnoll wights...
Anyway, I'm fully capable of customizing my monsters, but I wanted to know if there was a more scientific method out there for it. Thanks, Set.