Golduck? On the fossil set card, its Pokedex data said it was usually mistaken for the legendary japanese monster, Kappa. And it swims in rivers... and possibly swamps?
Paras? It is parasitic like mistletoe, which symbolizes love, which symbolizes hearts. It also has TWO mushrooms on its back. And it's a bug, like a caterpiller. That's all the clues that I can think of. (please be right)
Feebas?
(1) People were getting confused how to make Feebas evolve.
(2) fish eat caterpillers
(3) Its body looks like it's full of bubbles
(4) It can live in seas and in rivers?
(5) It just needs a little love
(6) I don't know