The last time I checked, you can't lock game mechanics behind patents or copyrights, and being able to ride a flying creature hardly started with Pokémon. For that matter, none of Pokémon's mechanics are really unique to that franchise: the Genie In A Bottle trope comes from Arabian Nights, which itself is a translation of stories dating back to the 8th Century.
So one could easily demonstrate prior art going back 1200 years... I think that more than invalidates the "capture a creature in a ball" mechanic.
As to summoning beasts to fight for you: Dungeons & Dragons would like to have a word with Nintendo.
