Hello, Candy and welcome.
I'm so sorry your cat ate your dog's 20 mg. Prozac capsule. The usual dosage of this medication for cats is 2-5 mg. a day, so even using the 'outside' number of 5, your cat ingested 4 times the usual amount.
If he just ate this capsule within the last hour, you cat try to induce vomiting according to the directions given, here:
If you don't want to try that or you are unsuccessful, bring him to a vet or ER vet now, so he can be evaluated and treated.
Side effects and signs of overdosage are the same: Anorexia and behavior changes (anxiety, irritability, hyperactivity/insomnia and elimination behavior) are the most commonly reported side effects in cats. Seizures are also possible.
He might be very sleepy or very hyperactive. He may stop eating, not sleep and be anxious or irritable.
Since the amount he ate was an overdose, I'd recommend a visit to the vet so he can be given something like activated charcoal to bind the medication and absorb it so not a lot is absorbed by his body. Supportive care will be given, if necessary.
I hope all will be well with your furry boy and please let me know how he's doing. Thanks!