The only use of both items (Dead Frog (Item) and Dead Snake (Item)) are for decoration.
Chief Grarkharok can be fed these items :) He only likes fresh ones though. Whether or not it actually does something for the Troll Sabotage quest is not confirmed, so it appears to be just roleplay for now. In Mythology, Trolls are known to eat weird things so this could be why. I don't think any NPC buys dead snakes or frogs, I checked multiple fansites and they don't list this information.
Introducing the ability to bump and pin questionsJanuary 09, 2022
Fourth Anniversary InfographicDecember 30, 2021
Interview with Laethye - Game Content Designer of TibiaNovember 23, 2021