I have a 17lbs male dog that accidentally took my girl 5mg Prednisone tablet. Is it dangerous or life threatening?

Yes. Call the vet or local animal poison control center immediately so they can give you expert advice on how to handle the situation and whether your dog needs to be seen by a vet. This is based on the product packaging warnings I could find online:

KEEP OUT OF REACH OF CHILDREN. Harmful if swallowed. May cause gastrointestinal irritation.