A Ditto will appear in the wild for everyone, and isn't just a common Pokémon that can transform at random - so if you catch one, other players nearby can get the same one too.

Do dittos look different in Pokemon go?

Although it may not look like much, Ditto is capable of mimicking the appearance and powers of any other Pokémon it sees.

Are dittos worth keeping?

Outside of this really niche circumstance, Ditto is not very useful. Most of the time, it will be copying movesets that won't even hit the opponent hard, since most types are not very effective towards themselves.