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Pokemon need this universe's physical laws to be broken to exist.
Some massive autistic reasoning:
>Normal, Flying, Bug: Feasible.
>Poison: Discutible. Definitely there are animals that use different kind of poisons to defend or hunt, but cases as Grimer and Muk would be impossible unless you interpret them in a fancy way (for example, Koffing being a fungus)
>Fighting: Not feasible the way it's conceived in Pokemon world.
>Water: Barely feasible, but Hidro-pump would break easily the Mass Conservation law.
>Fire: Barely feasible, same case than Water, but replacing water with fuel. Plus these beings would need an igniting agent.
>Grass: How animal and plant cells can coexist in only one multicell being is beyond my knowledge. If it is a mutualism case, it would be feasible (would be the case of Bulbasaur and Turtwig lines); however most Grass attacks would not, as they need mobility that plants lack (i.e. Frenzy Plant, Ingrain, etc.)
>Psychic: If theories like Morphogenetic Fields are proven to be true, it would be feasible. Sodium/Potassium channels make ions move, meaning that they would generate a tiny magnetic field. If there is a way to interpret these...
>Ghost: Ghosts don't have the matter necessary to work as living beings.
>Ice: Cooling with laser is a thing right now, but in atomic scale.
>Fairy, Dragon, Dark: Folklore based.
>Rock, Steel: They would have serious body temperature and mobility issues. Their skin would be way too heavy for them to move properly. However corals exist as living beings with a hard shell.
>Electric: They would need a body structure like a capacitor, isolating loads of positive and negative charges with every other body part. Only contact attacks would work, like Thunder Punch and Volt tackle, as the target would need contact with both "terminals" for the attack to work.
Now, if it's in a Virtual Reality world, audiovisual aspects can be easily simulated.
Pic just for fun.