From Zelda Legends Wiki
A wooden creature with leaves anologous to human hair. It has glowing red-orange eyes and tree knotholes for a mouth. They genarally keep to their own kind but some are friendly.
Usually it will pop out of it's hole and spit nuts at people. Hitting it with a sword or knowing the nut back with a shield will stop the Deku Scrub.
Deku means wooden doll in Japanese (or wimp), and scrub is an english word for low lying bush.