Werewolves can be bitten by vampires, and vampires can be savaged by werewolves, either of which will result in a vampire/werewolf hybrid. Why would one need a mod for this?
Beats me - you can definitely have were-vamp hybrids in-game. (In fact, I believe you can have an alien-plantsim-werewolf-vampire-witch-zombie, although I've never bothered; sunlight can be a bit of an issue for the plantsim-vampire, but I know people have managed it.)
I was both impatient and curious enough to try to go figure out what the heck Nevermore meant, so I ran a search. There's a "Vampire and Werewolf Blood" mod on this site that will provide a shortcut for becoming a hybrid: basically, buyable potions that will let you become a vamp or a wolf without needing to be nibbled by the LotP or bitten by a Grand Vamp. (If that's the mod, I'd recommend using the Sim Transformer instead - it's newer and has ALL the supernatural options on one painting.)
There's also a werevampfixes on MATY, but that was more of an annoyance-fixer, IIRC.
My only other thought was of a possible fix for a clash between a werewolf skin and the vampire overlay, but I think the only time there was a problem with the werewolf skin having conflicts was when there was a custom skin in use - and since the vamp "skin" is an overlay, not a custom skin in and of itself, I don't know that they'd clash anyway. Someone with more experience would have to answer that; I only have witch-vamp hybrids in my own game at the moment.
Also, slightly OT: MTS2 does have some very fun vampire mods. I currently use no-skin-overlay daywallking vamps in-game that I modded to display normal thumbnails (the daywalking does make them overpowered, admittedly, but I have a preference for more human-like supernaturals) but there are a couple specific mods and pieces of CC I'm thinking of: Black_Spirit has some more universally useful mods that allow vampire biting to either kill other Sims by biting, feed off Sims (satisfy Hunger) without killing or changing them, or turn a Sim into a vampire, and Fanseelamb also has a Tru Blood custom food for vamps if you want them to drink synthetic blood instead of using a mod to feed.
The vampwerefixes mod is actually the first mod I ever downloaded (first CC of any kind, in fact!), because I'd heard of the awfulness that results if Mrs. C is vamped, and this mod prevents that (among other things). It has since been replaced by creaturefixes.
I don't have a lot of mods for vampires, but am aware of many of them. They're kind of in the back of my mind as things I might want someday, but for now, I've just got default replacement vamp skins that simply pales their normal skintone slightly, and leaves their eye color the same as when they were human. I don't mind the default vampire look, really, but this is a bit better.
As for hybrids, I've had vampire plantsims a number of times. Plantsim witches too, but I don't really consider witches to be "creatures" so much as normal sims with magic skills. Thus, I can't bring myself to think of a witch who's also one creature as being a "hybrid".
But anyway, yes, many of the creatures can be combined. Obviously, base sims (humans, aliens, bigfoots, servos) can't become each other, but any of those can become werewolves, vampires, plantsims, zombies all at once. (Well, except I'm not sure if bigfoots and servos can be plantsims?) Sims like this are referred to by some as "Black Sheep", which I THINK is a phrase Pinstar coined for his Legacy challenge.
Making a black sheep is one of those Sims 2 projects I've been thinking about for a long time, but haven't actually done yet.
Ah wooooooooooooooooooooooooooooooo don't ask. I thank everyone for helping me the wolf thing. I keep seeing the pack leader now. >.> He or she comes out a lot more now -.-" But its okay, one day, I will enter the darkness of the night, and howl at the moon. One day.
"Queen of the Damned seeks knight in shining piercings for pleasure, pain and purring"--Scary Mary from the Urbz: Sims In The City
"A Famous Explorer once said: 'The extraordinary is in what we do, not who we are.'"--Lara Croft from Tomb Raider 2013