who would win

One billion Pokemon, Kyogre would probably solo by sinking all of the lions down. Palkia and Dialga can just mess with space and time and destroy the lions without much effort. Giratina can probably solo in the Distortion Realm. Arceus can probably just erase their entire existence before they were ever born. Yes, Pokemons, especially Legendarys and Mythicals are over-powered.

If you really mean every lion... all it takes it's a single egyptian Maahes lion god, since it's an "immortal" one would be enough to destroy every mortal being, now imagine what a billion lion gods could do... add some more lion gods like the african Apedemak lion god, indian Narasimha lion god, etc.

https://community.pokemon.com/en-us/discussion/comment/14782#Comment_14782absolutely love this, spot on.

https://community.pokemon.com/en-us/discussion/comment/14773#Comment_14773@DoubleCure You do realize that Arceus and the Creation Trio (Dialga, Palkia, and Giratina) are comparable to gods, and practically are gods. This includes being immortal: The Arceus in Legends: Arceus is likely the same one that was born in the beginning of the Pokemon multiverse.

it really depends on the pokemon, one billion pokemon doesn't mean every pokemon while one of every lion would be every lion that ever existed, so if it was one billion magikarps or caterpies then the lions could just eat them all.