Now George Noah's Ex-Zack-ly What Happened (2) - S3-E28
Continuity mistake: The beginning of the episode continues right from the previous episode. George and Angie are wearing the same clothes but in this episode, Angie's hair is suddenly styled differently and she is wearing more make-up.
Answer: His last appearance was in the episode "God Needles George. He had a tumor in that episode, but in the end was better, and that was the last appearance of him on the show. He was occasionally mentioned afterward, so it is assumed that they still had the dog for the rest of the series.