(like if someone hits me as a giant ape, will they still take the cold damage?)
— ℙ ℝ ℂ ℍ (@Tris10_outta_10) May 17, 2020
Armor of Agathys (PH 215) and Polymorph (PH 266) do not make exceptions to each other. Since temporary hit points are separate from your actual hit points (PH 198), they would not be replaced by the statistics of the new form. Yep, existing spells on you when you polymorph are still there in your new form.
— Dan Dillon (@Dan_Dillon_1) May 17, 2020