The Most Popular Species
What furry species are the most popular? You'd think this would be an easy question, given how well-organized species tags are on e621. Just count the number of posts for each species and spit out the results, right? Well, here's what that would look like:
How incredibly disappointing and unsatisfying! Obviously mammal is going to be the most common, it includes so many different animals! We might as well say "animal" is the most popular furry species.
The cause of this frustrating result is e621's tag implications system. When somebody tags an image on e621 with poodle, this implies that it's a domestic_dog, and so that tag is also added. Implied tags can have implications of their own: domestic_dog implies the genus canis which in turn implies the subfamily canine. The implications system creates an entire tag taxonomy, with a huge number of tags ultimately leading up to mammal. Some of these can become quite complex:
border_collie ➔ collie ➔ sheepdog ➔ herding_dog ➔ pastoral_dog ➔ domestic_dog ➔ canis ➔ canine ➔ canid ➔ mammal
In order to produce a more sensible popularity ranking, we need to filter out some of these tags; some need to go for being too broad (e.g. mammal, feline), while others need to go for being too specific (e.g. nile_crocodile and saltwater_crocodile should be dropped in favor of crocodile).
Admittedly this is more art than science. Should red_fox and fennec_fox be treated separately, or just grouped together under fox? Should husky roll up into domestic_dog? Are vampire and robot even species? I don't think there's a right way to do this—no matter what, it's going to be a little arbitrary. Ultimately I tried to follow common sense while ensuring the tags were mutually exclusive, i.e. that none of the filtered tags implied any of the others. If this bothers you, you can still see the unfiltered species by clicking the toggle.
And there you have it. The most popular species is fox. Surprised? Yeah, I didn't think so. Of course, there are a few interesting findings; I wasn't expecting rabbit to be so high, nor eevee to be more popular than most of the other eeveelutions.
Of course, most popular overall doesn't mean most popular today. It might be interesting to look at these tags over time. The below graph is pretty busy, but you can remove lines from it by clicking on the tag names in the legend.
Sorry foxes. The future belongs to the domestic_dog. We actually see a few other interesting spikes in this graph, corresponding to the 2016 release of Zootopia and the 2010 release of My Little Pony: Friendship is Magic.