Of course, there is a third ‘extra variety’ of pokemon, though it is a bit of a catchall. This variety of pokemon exists largely due to some design choices the creators of the game made- one above all the others.
The later a pokemon evolves (or the more difficult it is to evolve the pokemon), the greater its power increase from the evolution.
Because of this, there are many pokemon who, even though they evolve the same number of times as other pokemon, simply don’t have high enough stats to compete with the frontrunners. A large block of this variety is composed of most of the pre-generation-four bug pokemon, many of whom evolved twice by level 20... and subsequently became mostly useless around level 30, by comparison. This is not to say that these pokemon don’t have merits of their own- they just aren’t strong enough in their strong points to be competitive.
The rest of this variety are the ‘novelty pokemon’, pokemon who don’t fill a strategic spot, and who were often included just as an element of the collecting aspect of the game, to draw people to play more in order to catch them. Luvdisc, Dunsparce, Yanma (before it gained the ability to evolve into Yanmega), Farfetch’d, Delibird- all of these are (or were when they first appeared) essentially there to be chased after for pokedex completion or just for the sake of it.
These pokemon are not, however, inherently useless. Dunsparce, when used properly, can use status-effect moves and flinching to lock things down, and while Beedrill doesn’t really compete with the sweepers its stat spread somewhat imitates, it can come out of nowhere if you’re not prepared for it and blow a hole in some Dark or Psychic pokemon with its Bug attacks.
Generally speaking, you aren’t going to want to use these pokemon- but that doesn’t mean you can’t, or that they can’t be used effectively. You are going to find yourself making a lot of concessions on your choice of things to team them up with, though, and there will be a lot of times where tehy only have one effective strategy which isn’t always a good one for the battle you’re in. Still, their appearance on your team can serve as an interesting and distracting surprise for your opponent- especially if they’re used to seeing the more common pokemon in use and don’t remember what works best against them. Even compared to the specialized or hybrid pokemon, you have to use these sparingly- but when you do and you use them right, you can gain a powerful (or just plain weird) advantage in mid-battle.