Niantic have not announced information about legendary Pokemons yet. However, many of leaked or hacked videos are on the internet. This page introduce on of those videos. It shows a Ditto, the specie is not found as well as legendary Pokemon, in a gym battle (3:30 - 4:15). The Ditto defeats three defenders: a Magmar, a Gyarados, and an Arcanine.
The CP of the attacker is the number next to the names of Pokemon.
The first defender is the Magmar and its fast move is Ember. It is not sure that which fast move the Ditto uses, but it seems to be Pound, a normal-type fast move.
The Ditto defeats the Magmar by using the fast move four or five times. It is weird but can happen because the video seems to be intentionally created by someone as like an entertainment.
Ditto vs Gyarados
The next battle is the Ditto vs the Gyarados. The Gyarados's fast move is Dragon Breath. The fast move is not very effective against the Ditto even though normal-type Pokemon like Ditto is usually not strong to dragon-type moves.
Like the battle 1, the Ditto beats the second defender by only using its fast move. The move is so powerful and fast.
Ditto vs Arcanine
The third defender is Arcanine, and its CP is higher than the attacker. By using charged energy, the Ditto uses Psychic, psychic-type charge move.
The reason is not sure, but the Ditto's fast move significantly damages opponents sometimes. The Arcanine is about to use Fire Blast but is defeated before using the move.
How to beat
Tap the name of each Pokemon to see the best Pokemon to beat it.