Sopdet (Sepdet, Sothis) personified the 'dog star' Sirius. This star was the most important of the stars to the ancient Egyptians, and the heliacal rising of this star came at the time of inundation and the start of the Egyptian New Year. As a Goddess of the inundation, she was a Goddess of fertility. She also was linked to the pharaoh and his journey in the afterlife.
She was represented as a woman with a star on top of her head dress, or as a seated cow with a plant between her horns as depicted on an ivory tablet of King Djer. The plant may have been symbolic of the year, and thus linking her to the yearly rising of Sirius and the New Year. She was very occasionally depicted as a large dog, or in Roman times, as the goddess Isis-Sopdet, she was shown riding side-saddle on a large dog.
Source: Sopdet, Goddess of Sirius, New Year and Inundation by Caroline Seawright