True to its name, the Thorn Bow is entirely composed of prickly thorns. A bloomed red flower and flower bud reside at the top of the weapon, along with another thorn growing in a distinctive direction.
Epic Battle Fantasy 4
Found on the Battle Mountain, specifically in the bottom-right chest on the Rainbow Rafflesia screen, accessible after beating the eight-wave foe rush with foes from Lankyroot Jungle. The Stepladder is required to get this far.