The 1970s Gold Key series THE OCCULT FILES OF DOCTOR SPEKTOR proves pretty rich in mashup scenarios, though not all of them qualify according to the categories I laid out here.
The main character is an expert on the supernatural who runs around investigating spooky stuff, even though he's not really a man of action and has no supernatural powers himself. After meeting versions of Mr. Hyde, the Frankenstein Monster, and the Mummy, and an "original" vampire antagonist named Baron Tibor, Spektor has an encounter with a group of vampires. Technically this can't be a mashup since all the vampires share similar origins, but it's worth noting simply because it teams up the best-known literary vampire, Count Dracula, with three others from prose lit: Sir Francis Varney (VARNEY THE VAMPIRE, Mircalla Karnstein (CARMILLA), and Lord Ruthven (THE VAMPIRE). Baron Tibor gets involved, but on Spektor's side.
The first true "monster mashup" in the series is issue 9, but it's an illusory one in which a witch causes Spektor to dream that several of his foes are attacking him (the "vampire" listed on the cover is the series' original creation Baron Tibor).
In issue #11 Spektor is afflicted with a werewolf curse, and in #12 he has a fight with the Frankenstein Monster that's clearly an homage to FRANKENSTEIN MEETS THE WOLF MAN. I can't consider this a strict "mashup" though, since Spektor is in the position of the main character and he's only temporarily a werewolf.
ADDENDUM: I've reversed myself on two things stated here.since posting this comment on a horror-forum:
What I called "the visual personality of the monsters" has given way (in my mind, anyway) to the character's history as a "central character" in his own narrative, at least when you're speaking of one-on-one conflicts.
So, like I possibly said before, FRANKENSTEIN'S BLOODY TERROR wouldn't be a monster mash, because it just pits its werewolf star against a "stock vampire," one "Count Janos." But if the vampire had used the name of a famous neck-biter like Dracula or Lord Ruthven, then that would be a mash, even though the vamp would still play the part of the antagonist, rather than that of a featured monster in an ensemble, like all the Universal mashes.
I have to look at the two films Naschy made where El Hombre Lobo goes up against a witch-queen. I remember them as being fairly similar, but WEREWOLF VS, THE VAMPIRE WOMAN just makes up a witch-queen out of whole cloth, while NIGHT OF THE WEREWOLF at least uses the name-- though probably not much else-- of Lady Bathory.
Another one that's out would be THE WEREWOLF VS. THE YETI, since the latter character would just be a "stock Abominable Snowman."
The difference between a "stock type" and an "artful imitation" is a tough one, too. The heroic monsters of MONSTERS VS. ALIENS are not supposed to be in any way identical with the movie-monsters on which they're based: the Fly, the Gill Man, the Blob and the 50-Foot Woman-- and yet, they have a textual relationship, since the informed audience-member knows what the hero-monsters are based on.
Matters of textual identity can go out the window, though, when you''re dealing with a alliance of monsters, whether or not they fight on the side of good or evil, or even if one of the monsters is largely controlled by the other (RETURN OF THE VAMPIRE).
By this line of logic, if Doctor Spektor-- even if he's only a temporary monster-- fights with a character who's supposed to be the original Frankenstein Monster, then that would be a true monster mash.
If this Spektor-werewolf also fought just one of the legendary vampires mentioned above, then that too would be a monster-mash. Spektor as a routine "ghostbuster" would not be a monster, so he gets no "mash-action" from battling Dracula or any of the others singly. But as long as he's facing at least two legendary monsters, that's another matter.