Pokémon GO players, you may (or may not) have already discovered this.
If not, then perhaps those Rattatas and Pidgeys that you’ve ignored may be a much more interesting catch. One of them might just be a Ditto hiding in disguise!
Ditto will only reveal its true identity after you’ve successfully caught it, so the only way to maximise your chances of finding one is to keep catching Pokémon!
In gym battles, Ditto will assume the form of the enemy Pokémon it first encounters, copying all abilities at the same time. It’ll stay like that throughout the gym battle too, which may or may not be useful depending on the gym battle!
Pokémon GO has been hitting a few successes lately, including winning the Golden Joystick for Best Handheld/Mobile game and Innovation of the Year.