Ditto
When Pokemon GO was first released, ditto, strangely enough, was the only unreleased Pokemon that wasn't a legendary Pokemon; however, since that time, Ditto has been released within gameplay. Unlike every other Pokemon in Pokemon GO though, Ditto can't technically be found in the overworld (wild). Instead, it can be found transformed as either pidgey, rattata, zubat, magikarp, hoothoot, sentret or yanma, with the trainer not knowing until after they catch the Pokemon. At that time, Ditto will change from its transformed form into its regular form and a trainer will be notified of such.
At Pokemon Gyms, Ditto will immediately transform into the other Pokemon it is facing. Ditto can't reach an astronomically high Max Combat Power; however, that doesn't really pertain to it. You see, after transforming when in battle, Ditto's CP will skyrocket and better imitate the Pokemon it is facing. It won't match the exact CP as the other Pokemon, but it will jump pretty close to such a number.