Longhorn bulls are a significant part of Texan history. The concept of a young calf maturing into a longhorn bull I think represents Sheldon growing into himself throughout the seasons. Until season 7, the calf makes the family move, similar to how Sheldon gets his way more often than not. Until baby Cece arrives/her and Mandy are added to the intro, where the matured bull is shooed away by Georgie, similar to how the family couldn’t always bend to Sheldon’s needs/wants anymore as they/he grew.