I mean it took us till Gen 5 to finally get a zebra and a hippopotamus Pokemon. Till gen 6 to get a goat Pokemon and a lion Pokemon everybody would recognize as such, as well as a t-rex Pokemon (particularly interesting when there's at least three sauropod Pokemon; Meganium, Tropius and Aurorus). Till gen 7 to a koala or a toucan Pokemon... Also till gen 7 to get a proper wolf Pokemon. Till gen 8 to get centipede and gorilla Pokemon.
Aside from a bottlenose dolphin (which is what people mean when they ask for a dolphin Pokemon, so people can really stop bringing up Kyogre) there's still several other "popular" animals that don't have pokemon yet. For example; we lack a peacock Pokemon, nor do we have a hummingbird (no, cutiefly is based on a bee fly) a manatee, a capybara, a narwhal, a housefly (you'd think that'd be the most basic bug pokemon), no leech, no mosquito, no earthworm, no swordfish...
There are heaps of well known and popular animals that aren't Pokemon yet, while some more obscure ones are. And there can be many reasons for that; maybe they liked other animals/plants/concepts better, maybe they want to pace themselves with the "popular" animals, maybe they wait for a really good idea etc. etc. etc.
Same with type combination. There's still no Bug/Dark type, for example after 8 generations worth of Pokemon and other type combination (such as normal/flying and rock/water) being re-peated ad nausem.