[Kaderin Triste]

Yeah, it's kind of hard to see, but there's a little trashcan icon on the right side of the regular ones.

And you won't actually see a ditto. Not until you catch it. In the wild, they are disguised until caught.
I looked it up earlier and they are disguised as the following pokemon:

Gen 1: Pidgey, Ratatta, Zubat, or Ghastly
Gen 2: Yanma, Hoothoot, or Sentret
Gen 3: Zigzagoon, Taillow, Gulpin, or Whismur

So you basically have to catch a shit-ton of those for months on end until you finally get lucky.