system
Given the following post, respond as if you're commenting on Reddit:
human
[Pokemon] What defines a Pokemon and what makes it 'legally' viable for battle?

What is a Pokemon anyway? And when is a Pokemon allowed to fight in a Gym battle?

Now, of course most Pokemon are wild animals. Pikachu is simply a mouse that has electric powers.  
It gets a tad more confusing when it comes to more humanoid Pokemon or Pokemon with intelligence similar or greater than that of humans.  
Then later down the line we have Pokemon that are heavily implied to be extraterrestrial. Pokemon like Deoxys, Clefairy and Beeheyem.  
Then, later down the line we start encountering things that aren't even animals or creatures, but paranormal phenomena such as the living doll Banette, born out of spite for being abandoned by its child. Or Yamask, which is just a formerly human ghost. Not to be confused with the ghost of Marowak fought in Pokemon Red, which is not a Pokemon but a Pokemon's ghost. Not even getting into how the entire Marowak/Cubone line seems to be a paradox.

But what really, really confuses me is the artificial Pokemon.  
Golurk, Porygon, Mewtwo, Genesect.  
How are these legal? Genesect is a crustacean with a cannon mounted on it. Is that really allowed by the official Pokemon League? Can I hand my Pikachu a gun and call it a new Pokemon? Can I send a drone into battle and claim it's a new Pokemon?  

At  what point does something become labeled as a 'Pokemon'?
gpt
This was actually explained in the manga.

Pokemon can bring tools if they are approved by the Pokemon league or used naturally. So man made items like Choice Band but also items they use naturally in the wild like Farfetch'd stick. A gun or knife would not be legal in an official league fight.

Also Pokemon that exist as a permanent or semi permanent group naturally like Exeggcute or Magnezone are fine, but duct taping two Gyarados together is not.

As for artificial Pokemon it's a bit less clear. It's hand waved by "league approved" but that gets a bit more complicated for Mewtwo and Genesect. Maybe Team Rocket has the league paid off with bribes and slowpoke tails.