Cosmo is a dog that belongs to Mrs. Augustus.
Cosmo appears to be an adult Borzoi (also known as a Russian Wolfhound). Nothing is known about Cosmo's past.
When Tai, Claire and Mrs. Augustus are at the Augustus home getting high, Tai goes out to get snacks. While Tai is out, Claire's mother starts talking about dogs, and a dog wanders into the room (presumably, Tai left the door open).
The Vet says Cosmo is a girl and held on to the dog for a week to give her owners time to claim her. Mrs. Augustus called dibs if Cosmo remained unclaimed and took ownership after the week expired.
Cosmo is featured in a guest comic by K. B. Spangler.