I recently noticed that the tag canine_pussy has significantly fewer posts than the tag canine_penis. Is this due to an undertagging of canine_pussy or is there just a preference for canine genitalia when it comes to penises but not vaginas?

probably a mix of both. anthro canines are more likely to have humanoid pussies, but knotted canine penises from what i've seen, but probably some are just not tagged correctly as well. ferals are less common but more likely to have accurate genitalia.

Canine penises are more acceptable and 'mainstream' (for what that means in furry) than vulvas. Cookies are also harder to position on anthros in a way that feels right and shows it off.

Animal penises in general are more popular than animal pussy.

animal_penis has 360k while animal_vagina has 53k.
If you compare the specific clade tags(like lagomorph_penis vs lagomorph_pussy), you'll notice a similar trend.

I'm sure undertagging plays a role to some degree, but I'll bet animal vagina just isn't as popular in nsfw art.