Aside from the Christmas Specials, Isis is shown at the very beginning of the show's introduction, walking beside the Earl of Grantham.
Series One Edit
Isis occasionally appears in the background in Series One.
Series Two Edit
Isis appears in Series Two when Mrs. Patmore feeds her leftover Crepe Suzettes after telling Ethel that none were left over from the previous night's dinner. She also made several background appearances.
Isobel also mentions her when she talks about the convalescent home, she asks what can be done about keeping her out of the rooms to which Robert replies "I can answer that - absolutely nothing!"
Christmas Special Edit
Isis appears in the Christmas Special when Thomas hides her in a shack. His plan was to keep her hidden and then find her to gain Lord Grantham's trust, potentially causing him to be promoted to valet. However, his plan backfires when a village girl finds Isis and brings her to the Earl for a reward. However, the Earl sees that Thomas was searching for Isis and is touched. Thomas gains his trust and is hired once again.
Series Three Edit
Isis occasionally appears in the background.
Christmas Special 2012 Edit
Tom Branson is holding Isis on her leash when the family leave for Scotland. Robert asks him to walk her a lot, saying she can get lazy. Later he is walking her when he goes to lunch at the Grantham Arms.