agree, yes. even if 1 kill adds the entry, so you can "complete" the bestiary, as far as entries and zoologist's rewards concerned, but you won't actually have the description, and other info, and, yeah, (almost) no one's going to ever hunt 50 of the super-rare monsters...
so, yeah, it would make sense to just have rarer enemies require less kills to add all the info to their entries. bosses complete their entries right away too once defeated, so I'm assuming it wouldn't be too hard to do with rare enemies either?