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.

See also

Other fiber / GI questions for dogs