Ditto hides as other common Pokémon until you catch one.
Reports are still coming from players about what creatures Ditto hides as, but many have caught the purple blob in the form of Pidgey, Rattata, Magikarp, Spearow, and Zubat.
Some have also caught Ditto from less common Pokémon, like Oddish, Paras, and Pidgeot.
Here's what it looks like when you catch a Ditto:
Managed to get another video of catching Ditto, this time it transformed from a Rattata @trnrtips #PokemonGO #ditto pic.twitter.com/s45wGgFnxc
— Nick from WhatUpMC (@WhatUpMC) November 23, 2016
Your Ditto will copy the moves of the Pokémon your battling and potentially have a higher CP depending on its health and your trainer level.
Ditto copies the move-sets of whatever Pokémon you're fighting, but it maintains its own health — including how much you've upgraded it with candies and stardust and your personal trainer level.
So even if your Ditto's CP is vastly lower than the Dragonite you're about to fight, it could become stronger than the Dragonite you're facing once it transforms.
The only catch is that it takes a few seconds for Ditto to transform, which gives the Pokémon you're fighting a chance to strike first.
Another catch with Ditto is that it remains the Pokémon it morphs into until it faints or all gym battles are over — even if you switch Pokémon mid-fight.