Caterpie is a bug type, known as the worm Pokemon, and known as Catapie in Japan. It can only learn one move through level up, which is bug bite. It can evolve into a Metapod at level 7, and then into Butterfree at level 10. When it evolves into a Metapod, it can learn only harden through level up, but once it evolves into Butterfree, it can learn attacks like gust, psybeam, and silver wind.
In the anime, Ash caught a male Caterpie, being the first Pokemon he caught. It soon evolved into a Metapod, and then into a Butterfree. Butterfree would also be the first Pokemon to let go, when it fell in love with a pink Butterfree, releasing him so he can start a family with the pink Butterfree.
Pidgey is a normal and a flying type Pokemon, known as the tiny bird Pokemon, and known as Poppo in Japan. It can learn attacks like gust, and whirlwind. Pidgey can evolve into a Pidgeotto at level 18, and then into a Pidgeot.
In the anime, Ash caught a Pidgeotto on his adventures through the Kanto region. Before Ash went to the Orange Islands, Pidgeotto evolved into Pidgeot to protect a group of Pidgey and Pidgeotto from a Fearow, and soon staying behind to continue to protect them, though Ash promised that one day, he’d return for it.
Nidoran is a poison type known as the poison pin Pokemon, and has the same name in Japan. Nidoran is unique for the fact that depending on its gender will it appear different. If it’s a male, it will be pink with long ears, and a long horn, but if it’s a female, it’ll be blue with short ears and a short horn. When the female evolves into a Nidorina at level 16, it looses its horn, but gains a long tail. When Nidorina evolves into a Nidoqueen through the use of a moon stone, it gains a short horn again. When a male Nidoran evolves into a Nidorino at level 16, its horn and ears gets longer a bit. When you use a moon stone on Nidorino, it evolves into a Nidoking, gaining a long tail, its ears become more round, and its horn a little longer. Nidoran can learn attacks like horn attack, and poison jab, but once they evolve into Nidoqueen and Nidoking, they can only learn four moves through level up.