is that b/c some are going extint or b/c we're discovering new rodents/other mammals?

Well Rodenta (aka rodents) is the largest family of mammals and its give or take 40% of all known mammal species; but this includes things like beavers.

but you are comparing two values, number of spieces vs number of population.

Genius Homo has 1 species (or at least only one recognized) but has 8ish Billion members Homo Speians (aka Humans). Genius Panthera has 5 species (Lions, Tigers, Leopards, Jaguars, Snow Leopards) but total population less than 200k. Thus one measure (number of species) Panthera is 5x as numerous as Homo. However if you look at population Homo is 40,000-50,000x as numerous as Panthera.

There are a LOT of bat speicies and lot of rodenta speicies but most aren't that populous (at least compared to say humans).

There are a lot of species of beetles much more than the number of ants, however the most ant species are very big in population.

Thus if you were to look at Species the animal world is basically mostly arthropods. But if you look at population its mostly worms.