Baby Pokémon were first found in the Neo: Genesis series and continued until Skyridge. They have 30 HP but there is a rule that if they are the Active and your opponent tries to attack them, your opponent flips a coin and if tails, the attack fails. They can evolve into what was a Basic Pokémon. The really good ones, Genesis Cleffa and Discovery Tyrogue were staples in many decks. They do count as Basics.
With R&S, they are no longer Baby Pokémon just Basic Pokémon with a Power that let's them evolve into the Basic (and remove any damage counters when doing so).