Wall Creeper

The Wall Creeper is a spider-like enemy that can be found in caverns, spawning in and around Spider Caves, mostly in pre-Hardmode worlds. Wall Creepers are capable of climbing background walls as well as through platforms, but revert to acting like a fighter enemy when there is no background wall.

Trivia

 * A quote of the Skeleton Merchant suggests that he may have become skeletal due to this spider.