No, they can't normally disguise themselves as others of their species
Ditto, Zorua, and Zoroark usually always disguise themselves as other Pokemon. However, although not necessarily disguising them as others, if you find a Ditto/Zorua/Zoroark, save the game and reload it, Ditto, Zorua, and Zoroark will appear undisguised and encountering them will make them do their reveal animations, even though they aren't disguised as something else. So technically, they can disguise as a Ditto, Zorua, or Zoroark but only themselves, never others of their species.
I think it's easier to see this with their shinies. Let's say we have a shiny Ditto. The Ditto would transform into a non-shiny Pokemon of another species, but when you reload, the Ditto will be a shiny Ditto, not a regular Ditto, if that makes sense.
Source: Experience.