Mandragora

Mandragora (マンドラゴラ) are a magical and type of living plant that is used as a universal ingredient for making potions.

Description
Mandragora is a humanoid-shaped root-based plant that is used as a universal ingredient in the making of potions. To be used as an ingredient, one must silence their screams, so that when it falls asleep you're then able to prepare it as a potion ingredient. If you don't soothe them and allow them to continue crying they will begin to grow larger and become considerably louder.

Introduction Arc
Lance teaches Mash and Finn how to make a Mandragora fall asleep so that they are able to pass an assessment. During their training, Finn was able to successfully make his Mandragora fall asleep while Mash made his Mandragora grow much larger due to not being able to use magic. He later successfully makes it fall asleep by slapping it causing it to be knocked out.

Trivia

 * Mandragora are likely based on the mandrake plant from Harry Potter.