Okay, going down the list . . .
Alien - Conehead, always and only, because that's what they are and that's what they look like. I cannot see any other options here . . . well, other than Coneheads and Preservers. Though Preservers are more Pixie-like Faeries than the classical understanding of Aliens based more or less on the Grays.
Devil - Uhhh . . . not sure here, because there are so few evil tempters in the series, much less vicious evil tempters who wear predominantly black or red and are associated with goats . . . though if I had to choose one, I'd go Haken. Traitor against the others, outcast from the group because of pride and arrogance and inner darkness . . . I can see the arguments for him.
Dragon - If you use anything other than Madcoil, you're missing something. That creature is the only Dragon-ish thing in the series. Even if it is it's own form of chimera. Now, dragon in mortal form would be the Dominance from the Shards series. Now *there* is a hoarding monster of a creature. If he can't have it, can't nobody have it? Yeah, that's Dragonish behavior as understood by Western culture. If you're going more the Eastern cultural understanding of the Dragon as an emblem of balance and leadership and all those good qualities then you'd probably be looking at Cutter, almost. It depends on whether you want to focus on the physical appearance of the Dragon or on the mystical traits carried by the emblem.
Frankenstein - Honestly? Two-Edge. You have to remember, in the original books, Doctor Frankenstein was more monstrous than his creation, but we always call the monster by his name. Frankenstein's monster was a genius and was tormented by his unnatural creation, always seeking to destroy his creator. (As I understand it from descriptions by others. I haven't actually read the book by Mary Shelley.)
Ghost - Could be any of those who have passed the boundaries of death and returned as spirits to aid or harm the living. Zahntee, especially. Also Winnowill's spirit held captive in Rayek.
Hellhound - Uhh, Peacehound or Blackfell, though the more conventional understanding of the role of the Hellhound would mean leaning more towards the Peacehound than the other. They're supposed to be frightening and demonic.
Black Lagoon Creature - I can see the argument for the Broken One, though there's a pathos to the Broken One that just isn't there for the Creature from the Black Lagoon which is more Mer-Winnie or even Surge. I'm leaning towards Mer-Winnie, though, for sheer imagery. Remember the two-page spread from Kings of the Broken Wheel?
Mummy - Anyone in wrapstuff who continues to affect the outside world, from Orolin to Timmain to Ahdri
Skeleton - Hmm . . . difficult one to say, there are so few good animated corpses in the series. There's the one human that Winnie has in her basement, the one who's waiting to die, the one she shows to Suntop to frighten him with his tribe's mortality.
Vampire - Uhm, lessee, king of the Undead, life-sucking darkness, super smexy . . . yeah, Winnowill or Voll. Not to mention, Voll dies by being impaled by a shaft of wood . . . through the chest . . . After rising from his coffin . . . Uh . . . yeah.
Werewolf - Kimo or Timmain although Bearclaw presents a fascinating option for the more untamed aspects of the role as opposed to simply the shapechanging ones. I guess it depends on whether you like your Werewolves to lie by the fire and let you cuddle them or whether you want them shredding the couches and howling at the moon.
Witch - Uhh, gonna go human for this, but Bone Woman. She fits the classical concept of the witch, treating with dark powers to gain authority over others and all that stuff. Admittedly, it's one interpretation of the term, but still . . .
Zombie - Uhh . . . again, dessicated corpse animated for nefarious purposes? Any of the walking undead of Blue Mountain or the human Winnie kept in the depths who was simply sitting waiting to die. Could also include either Door, Brace, or Egg.
Just my opinions added to the mix.