Dipping a corpse into a potion of polymorph “keys” the potion to that specific monster type: if it is used to polymorph the player or a monster, they will be turned into the same monster type as the corpse used. The corpse and potion are not consumed by being dipped.

If the monster is no-polymorph, nothing happens.

If the potion is used to polymorph an object, it does not do anything unusual, unless the object is a type for which species matters, in which case the potion changes its species to the keyed type, which can be useful for changing a figurine to something specific.

If in a variant that has potions of specific monsters’ blood, this can also be created by dipping polymorph into monster blood or vice versa.