Complex sentence: Stanley was introduced in "The Great Discovery", and has roles in Seasons 12,15 and "Day of the Diesels" as well as cameos in "Misty Island Rescue" and Seasons 14,16 and 18 and "Blue Mountain Mystery".
Split into simpler sentences: Stanley was introduced in "The Great Discovery", and is featured in Seasons 12, and has appeared in every season since Season 14, (with the exception of the 17th). He has also appeared in "Misty Island Rescue", "Day of the Diesels" and "Blue Mountain Mystery".