are made of these~
I've RPed several times with Legendary Pokemon plotstuffs, in whatever way. (Receiving Legendaries, getting the power of Legendaries, and currently having been a Legendary in a past life. ) It can be incredibly fun!
But like PkmnTrainerYellow said, you have to be careful to limit their powers or choose Pokemon. For example, in one of the RPs, Celebi didn't have its time travelling capabilities because that would seriously mess up an RP.