In Pokemon Black and White there are two different grass-type lines that are exclusive to either game; you can only catch Petilil and its evolved form, Lilligant, in Pokemon White, and Cottonee and Whimsicott in Black. However, there is an in-game trade that allows you to obtain the first stage of the line that you can’t catch.
In the pokedex, Cottonee and Whimsicott come first. Cottonee appears to be based off from a ball of cotton. The star-shaped pattern on its back brings to mind cauliflower, as does its folded leaves on each side of its body. According to its pokedex entries, Cottonee drifts with the wind and goes wherever breezes takes it. Whimsicott, on the other hand, seems to be based off from a creature from Brazilian folklore that play practical jokes, ride wind currents, and inhabit forests. Whimsicott could also be based off from the Vegetable Lamb of Tartary, as the author believed that the sheep-growing plant depicted in his novel explained how cotton existed in the Middle Ages. According to Whimsicott’s pokedex entries, it rides on whirlwinds and can manipulate its body to fit into any size crack. It uses that ability to sneak into houses and cause all sorts of mischief for the house’s inhabitants. Whimsicott’s name most-likely comes from the words ‘whimsical’ and ‘cotton’. Both Cottonee and Whimsicott have a 50/50 gender ratio.
After that evolutionary line comes Petilil, the Bulb Pokemon. Petilil appears to be based off from an onion or a flower’s bulb, such as a tulip. The latter would explain the corruption of the word ‘petal’ in its name. The ‘lil’ in its name comes from the shortened version of the word ‘little’. According to its Black and White pokedex entries, Petilil are known to reside in areas good for growing plants. The leaves atop its head are supposed to be bitter, though those leaves can refresh worn out Pokémon. Petilil’s evolution, Lilligant, has a much more complicated design. It appears to be wearing a dress of some sorts, most-likely based off from the ones worn in the late 1700s, and has a hat-like flower with a crown on top of its head. Lilligant can also be based off from a dancer as it can only learn moves with the word ‘dance’ through leveling up. According to its pokedex entries, even long-time trainers have a hard time at making its flower bloom. Because of its beauty, Lilligant tends to be popular with celebrities. Lilligant’s name most likely comes from the words ‘lily’ and ‘elegant’. What makes this evolutionary line unique, though, is that Petilil and its evolution, Lilligant, are a female-only species.
The two game exclusive grass-type lines in Pokemon Black and Pokemon White are quite unique. Not only can each line only be caught in one game, but both lines require the Sun Stone to evolve into their final forms.