I am reckoning that I only need 5 of these monsters at the most with proper runes on, as I will never need more than that (apart from all-nature wars) so I am thinking of adding some gold runes to increase productivity.
I have these Nature monsters at level 100:
Living Forest (only 1 rune slot)
Alpha Cliviast (only 1 rune slot)
El Dino Volador
I also have Drekk, Nemestrinus, Roastie, and Lagerchaun who are only level 50 and are used as gold slaves already.