Can I Give My Dog Metamucil?

Yes, with care
Psyllium husk is commonly referenced for dogs at published doses. Published guidance for dogs: small dog: 1/2 tsp; medium: 1 tsp; large: 2 tsp, mixed in wet food with water available, every 12-24h. PLAIN unflavored only. Sugar-free versions may contain aspartame or xylitol, xylitol versions are dangerous.
Source: Veterinary Partner (VIN)
Get the exact range for your dog's weight

Skip it or ask a vet first if

xylitol-sweetened productsintestinal obstruction

The safe habit

Confirm any medication decision with your veterinarian, who knows your dog's health history and other medications. VetDose shows published reference data, not prescriptions.