Mythical zoan's still the rarest though. There are only like 10 in the manga after over a thousand chapters.
Blackbeard giving it to a horse doesn't make it less rare, that's simply a choice that suits the recipient and amps them as well. The Blackbeard Pirates specifically hunted for devil fruits so them displaying two mythical zoans isn't crazy for an Emperor Crew actively looking for impressive fruits.
Kaido, Yamato, Orochi and Onimaru were the only mythical zoans from Wano despite the animal theme. So basically a whole Emperor crew comprised of plenty of zoan users didn't have any mythical ones aside from Kaido himself and his child. Sounds pretty rare and the other two were native to Wano, one of the largest islands we've seen the crew encounter and for the sake of the cultural background, it's understandable Orochi's fruit was here for instance.
Marco's the only one from Whitebeard's crew and we have no indication that Big Mom's crew has one and the Navy's only shown Sengoku so out of these major powers that would benefit from these abilities, only less than 10 have been shown and the others were from different islands.
As the series progresses and we see more prominent figures and their subordinates at play, it makes sense we'd see more of even the rare fruits. If mythical zoans surpassed logia in number then maybe I'd get the complaint but as of right now, the number of mythical zoans is pretty small and considering the Straw Hats encountered have encountered most of the major players, it may not even reach 20.