There are a couple.
Some of the obvious and more gimmicky ones include Ditto, Zorua, and Zoroark. These Pokemon can transform and can do that easily. I guess on a lesser spectrum, Mew could do that too, as it learns Transform, but I don't think there is any official confirmation of it changing into a human.
There is also Latias in the third Pokemon movie. The entire movie she is in the form of a Latias, but in the end transforms into a girl. It is unknown if Latios has this power, (or if any other legendaries have this power.
There are also examples of humans turning into Pokemon.
In Kadabra's Fire Red Pokedex Entry.
It happened one morning - a boy with extrasensory powers awoke in bed transformed into KADABRA.
Gengar's Moon Pokedex Entry states it used do be human.
It apparently wishes for a traveling companion. Since it was once human itself, it tries to create one by taking the lives of other humans.
Yamask is said to hold a mask of it's past face
Each of them carries a mask that used to be its face when it was human. Sometimes they look at it and cry.
Cofagrigus
This Pokémon has a body of sparkling gold. People say it no longer remembers that it was once human.
And according to azureshiram
the Latias from Emerald Arc Pokespe that is a friend of Emerald "Em", the main protagonist of Emerald Arc.
If there are any more that I may have missed please let me know.
Hope I could help!
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