A pokemon that has an initial moveset, then later had a lower pokemon added to its evolution chaing would be considered a baby pokemon. Smoochum was added in generation II, even though Jynx was made in Gen I. Machamp, however, had his evolution complete the whole time. Machamp wasn't added in later.
For the most part,they can only be obtained by breeding or getting them from an egg.
The baby pokemon are Pichu, cleffa, igglybuff, togepi, tyrouge, smoochum, elekid, magby,azurill, wynaut, budew, chingling, bonsly, mime jr, hapinny, munchlax, riolu, and mantyke.