I’d also add TNG’s Night Terror’s and Identity Crisis. Geordie turning into an upright-walking cat was just too much for that, especially since it was so spooky and mysterious before.
I thought he turned into a chameleon lizard? Using the holodeck to spot the mysterious shadow not cast by any of the landing party is one of the best “aha!” moments in the series.
I seem to remember cat-like eyes. You’re right though that the mystery was great, but I thought the reveal was just way too weird for me.