A seal was a type of aquatic animal originating on Earth.
In 1986, the Mercy Hospital on Earth kept many pictures on the walls of its corridors, including a picture of seals. (Star Trek IV: The Voyage Home)
In 2399, Kestra Troi-Riker had a drawing of a seahorse and a seal that was cutout and tacked or taped to her wall of her bedroom beside her bunk bed. (PIC: "Nepenthe")