Unfortunately (assuming you're using Google's Voice-to-Text engine) there's no way to add a personal dictionary. However, there's a weird 'workaround' that could work for you: add your cat as a contact! While personalized words aren't available, personalized pronunciation of contact names IS! Once you add the contact you can 'teach' Google how you pronounce that contact's name so it recognizes it properly.
Assistant helps you share and keep in touch with the people you care about. You can manage: Who to add to your people. How you’re connected. Their contact info. How Assistant should pronounc