Finn's Crossbow first appeared in Season 3 in the episode "Conquest of Cuteness". Finn's crossbow may be the crossbow that he stole from "City of Thieves". Also, in the episode "The New Frontier", Finn plans to use the crossbow to scare off the Banana Man to prevent Jake's Croak Dream from happening. After this, though, Finn doesn't use the crossbow often as much.
The crossbow has a dark wooden main-frame, but has steel on the top layer of the crossbow.