Prebutton

Before concluding the discussion of rattle width, we should make mention of the prebutton, the rattle with which every young snake is born, but which, unlike the true button, is shed and lost with the young snake's first change of skin a week or so after birth.

Source: wiktionary

All species of rattlesnakes are viviparous and are born with a laterally compressed keratinized prebutton on the tip of the tail. A few days after birth the snake sheds it skin and loses the prebutton, which is replaced by the first rattle segment, or button.

Source: wiktionary