Short answer
First, stay calm — a small taste of the juicy arils is usually fine. Your immediate step is to remove the fruit and check if your cat ate any of the bitter white rind or inner membrane, which are more problematic. For the next hour, ensure fresh water is available and watch closely for signs like vomiting, drooling, or loss of appetite, which can indicate stomach irritation from the tannins. If you see any of these signs or if a significant amount of the rind was eaten, call your vet for guidance.
Pomegranate — Food safety facts
Feed with care · Cats · full breakdown on the linked food page
A few arils are fine, but the rind and inner membrane are high in tannins and can cause vomiting.
The full verdict, portion guidance and serving notes are collected on the Pomegranate food facts page.