Is there any Pokemon ditto cannot use transform on?

Question

[besides a substitute ]

Answer 1

Transform fails if used on a Pokémon that is protected by Crafty Shield, but works on a target protected by other moves such as Protect or Detect. It also fails against Shadow Pokémon, if the user or the target are transformed or disguised by an Illusion, if the target is behind a substitute, or if the target's Ability is Good as Gold.

Transform will fail if the target is a Starmobile.

In Generation 8, a Dynamaxed Ditto in a Max Raid battle is unable to Transform into Zacian, Zamazenta, or Eternatus, as none of those three are able to Dynamax. However, outside of Raid Battles, Ditto is able to transform into these three.

Transform (Bulbapedia)

Comments

I’m pretty sure ditto can’t turn in to Eternamax eternatus, either

Answer 2

Ditto
An already transformed opponent (Ex. Ditto used Transform on Moltres, you switch into Ditto, but you can't transform into Moltres)
Zoroark with Illusion active - EC to fondant

Source

Comments

Thank you Le Scraf!

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That's on there

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Ditto can't transform into a Zoroark that has Illusion active.

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Transformed opponent, same diff.