Looking through many different articles about Pokémon, I feel that there are 3 main points to say that a Pokémon is indeed, a Pokémon. It needs to fit all of these criteria in my opinion to be a Pokémon. This is purely biologically related, so no Pokédex Entries that are coded related.
1. Any non-human sentient being found.
This one is pretty obvious. Pokémon are sentient, but aren't human. Now, you all might be asking me, "But what about animals that appear in the Anime/Cards and are mentioned in the games?" This is a valid point, and this is what brings us to our second point.
2. Their base form is able to be caught/obtained in a PokéBall.
Now, there are 2 main points to take away from this one. The first being base form. In this context, I am considering base form to be prior to any transformation not related to Evolution. This being said because of 1 main thing introduced, the lovely Dynamax/Gigantimax feature (Mega Evolution and Form changing Z-Crystals are still in the their changed from if switched out). All of these forms you can't catch and I can't say these aren't Pokémon. 2nd Point is obtained. Obtained meaning through breeding, events, etc. where you technically don't catch them.
3. It Needs to be able to learn some sort of Technique, or the more familiar term Move for defense or to complete a task.
This one ranges anywhere from Ditto's Transform to Mega Raquaza's Dragon Ascent. All Pokémon have it and this is mainly to prevent Devil's advocates from being like, "I shoved this cat into this Pokéball, hehe, it's a Pokémon.
Conclusion: These 3 characteristics are what I believe describes every Pokémon while still weeding out animals, humans, etc. This helps describe criteria for every Pokémon, but may have to be jointly changed over time to fit additional mechanics.
Sources for reference: https://m.bulbapedia.bulbagarden.net/wiki/Pokémon_(species)