Vines

Vines, also referred to as tendrils, are one of the dominant forms of life present in the Upside Down, and make up part of the Mind Flayer's hive mind; the vines are a form of life existing somewhere between the traditional idea of 'plant' and 'animal'. 
Will Byers was attached to a vine while he was strung up in the Upside Down equivalent of the Hawkins public library. Later, Jim Hopper was attacked by vines when exploring the tunnels beneath Hawkins; these tunnels themselves could be described as giant vines, containing smaller vines inside them. Furthermore, Vecna's true form in the Upside Down was connected to a series of vines extending from his back.