Freezing Potion A was first introduced in "Still." It is a potion used by Ice King to freeze Finn and Jake in their tracks. The only way to counter its effect is with Unfreezing Potion A. Finn and Jake used their sharpened brain sense to bring the potion to the Ice King, although they thought it was Unfreezing Potion A. It can be assumed that since there is a Freezing Potion A, there is at least a Freezing Potion B.
It is a green bottle with an attached piece of paper with the title "Freezing Potion A" written on it.