Dr. Dillamond didn't turn into a Winkie. He just lost his ability to speak (which is why the Wizard kept him hidden). And no, we didn't see him again.
You're right. The wizard kept Dr. Dillimond among the winged monkeys didn't he? Elphaba sees him after singing "Wonderful" with the wizard. Perhaps that's why carnation had him as a winged monkey.
And it was initially the wizard's monkey Chistery that Elphaba gave wings.
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