Alzrius said:
Gez, that isn't how it works. Anything WotC does is canon, then d20 companies, then fan-stuff
Not IMC.

It's a house-rule I made
More seriously, I would tend to say monsters from the ToH are more canon than monsters from the MM2 or BoVD. Why ? Why such a heretic train of thought ? How could I conceive such a blasphemous idea ?
Well, it's quite simple, and it stems from the very nature of D&D, and the very nature of the d20 license.
D&D is a generic RPG. Less generic than, say, GURPS, who intend on being applyable to any genre, but much more so than, say, Exalted, that is tied to one setting.
D&D in itself is merely a toolkit -- a collection of component among which you pick and choose to build your setting (you can also take a premade setting, like the Realms, for which most of the pick-and-choosing has been already made by other persons). This means that just because WotC has released the Desmodus in 3 books already don't force you to use them in your own campaign. It's modular. I'm pretty sure Loxos are not canon in the Realms, for example -- and I hope they'll stay non-canon; there are some things that are only acceptable, and even then, barely, in multiworld settings like DragonStar, PlaneScape or SpellJammer.
Even the true core components -- the content of the PH -- is not necessarily canon everywhere. A dwarven paladin would not be canon in a d20 Rokugan campaign (the default setting of OA), for example, but is perfectly canon in Greyhawk or the Realms. And yet an old-school purist could exhibit 2e or older books and claim dwarven paladins are not canon.
Here's for universal canonity -- it can't exist in a multisetting game.
To use a lingo that is surprisingly popular, fluff is canon for the setting it relates to, crunch is never canon as it don't exist by itself.
Now, I have not really adressed why the ToH would be more canon than a WotC book. After all, it has its share of useless monsters like all monster books do (keeping in mind that one's favorite may be another's useless trash). This leads us to the other point: the OGL and d20 STL.
Let say I'm a d20 publisher, and as such, I want to publish a d20 something. Let say a scenario/module/adventure, call it whatever you want. This ready-to-play story is rooted in folklore and include the crazened ghost of a powerful elven witch: a
banshee. I could either build my own monster, or avoid wasting time and using one already existing, but not the banshee from the MM2, not the keening spirit from the
City of the Spider Queen. But there is the groaning spirit in the ToH that just scream, pun intended, to find a place in my latest masterpiece. Now, in the sequels of that adventure (because it's a full-blown campaign I publish), the adventurers will discover a corrupted city of dark creepers, whose dark stalker leaders have been replaced by shadow demons, and finally discover that Orcus was behind all that and have to fight him in to thwart his nefarious plans (no harm done, he's used to that).
Dark creepers ? They'll be, from what it seems, in the upcoming Fiend Folio 3e.
Shadow demons ? They are in the Book of Vile Darkness.
Orcus ? Ibid.
But I can't use them. However, I can use those from the ToH.
In this way, the ToH is more canon, as more people are able to use it in
published material.