Unfreezing Potion A is the counteractive to Freezing Potion A and was introduced in "Still." It was supposed to be used to unfreeze Finn and Jake after the two had bonded with the Ice King, but is destroyed by Gunter. Finn and Jake used their sharpened brain sense to bring the Unfreezing Potion A to the Ice King but accidentally brought Freezing Potion A instead.
It is a green bottle with an attached piece of paper with the title "UNFREEZING POTION A" written on it. The Ice King underlined the "un" so as not to mix it up with "Freezing Potion A."