I actually have used a few of the animal stats, though i had to heavily massage them to have some semblance of realism, so i'm not sure whether to give the MM/D20SRD credit or not.
Roughly 0%, so that was the vote i used. But not for the reason you say--it's not that they're old and boring, it's that they're new and boring. The only creatures i would use out of the MM would be the dragons, orcs, hobgoblins, goblins, and maybe giants. All those invented things that have no basis in mythology or non-D&D fiction, i have no use for. Well, except Yuan-Ti, Githyanki, and Mind Flayers, which have always been my favorite badguys. But since none of those are OGC, i can't use them in anything i publish, and they're used minimally, if at all, in things that are published by others.
edit: forgot about undead. Historically, i've probably used less undead than most GMs, from what i hear, but still would use them a fair bit. For the current game, i'm trying to avoid using undead, for a variety of setting/mood/style reasons, and the only undead i'm using is the ghoul template from Fantasy Bestiary--much better fit for Arabic mythology than the D20SRD version.
And, even with those "traditional" monsters, the MM doesn't give me much to work with--just some stats, and i have to pull out the AD&D2 MC entry, or do the work myself, if i want any detail. So, why bother? As for the new invented things in the MM--none of them catch my interest, and, again, that's probably largely because they have basically nothing but combat stats and lousy art.