This is what a non-corporeal Patronus means:
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A non-corporeal Patronus can appear as 'a thin wisp of silver' that hovers 'like mist'. Whereas a corporeal Patronus has a form that is clearly defined and is more than vapour or smoke. The ability to cast a corporeal or non-corporeal Patronus is down to the skill of the witch or wizard.S
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That's why he was having a hard time producing an animal Patronus.