A Basic Pokémon is a Pokémon that has not yet evolved and is not a Pre-evolved Pokémon. They tend to have low stats, usually with an average of 2-3 stats rising one unit, and occasionally, one stat (speed usually) rises by 3. A Legendary Pokémon is not a basic Pokémon- as they are not as weak as a normal Pokémon. Pokémon that don't evolve (other than Legendary Pokémon) are still basic Pokémon. Magikarp is probably the most basic Pokémon around and the hardest Pokémon to evolve in the opinion of most. In the trading card game, they are also Pokémon that have not yet evolved and is not a Pre-evolved Pokémon. This has been part of the tcg ever since the base set. Many examples of basic Pokémon are:
1.It Will Evolves Into Stage 1 to Stage 2
2.It is Not a Baby Pokémon
3.In the anime, Pokémon like those above ARE NOT Basic Pokémon. They are JUST Pokémon.