From the Super Mario Wiki
M. Bush is a bush-like enemy found on Lavalava Island in Paper Mario. Much like Hurt Plants, they pretend to be ordinary bushes, but will ambush and attack Mario when he examines them. However, unlike Hurt Plants, M. Bushes can be told apart from harmless plants because of their stouter shape and lighter color. They have a low attack power and no defense, but they can poison Mario with their bite and usually attack in groups so they can be troublesome. The poison bite has around a 45% chance at having Mario to become poisoned.
A notable group of M. Bushes traps one of the Fearsome 5 in a tree. This group lies in front of the said tree, forcing Mario to fight them in order to rescue him.
Its name is most likely a pun on the words "ambush" and "bush."