galeanthropy
The mental condition of thinking that one has become a cat, which is usually manifested in the adoption of feline mannerisms and habits.
rare, uncountable
"That is the most interesting delusion I've come across since reading up on galeanthropy," said Sophie. […] / "What's galeanthropy?" I asked. / Sophie said, "It's the delusion that you've been turned into a cat."
Where it comes from
From Ancient Greek γαλῆ (galê), a contracted form of γαλέη (galéē, “weasel”) + ἄνθρωπος (ánthrōpos, “humanity”).
Adapted from the Wiktionary entry galeanthropy (CC BY-SA 4.0).