Mud (Earth + Water) + Dark = Poo.
Medieval (Fire + Metal) + Alpine (Ice + Earth) = Cool Fire Or Soccer Or Pearl Or Armadillo Or Flaming Rock
Neon (Dark + Electric) + Nenufar (Water + Plant) = Pirate.
Zombie + Mud (Earth + Water) = Petroleum.
Laser (Fire + Electric) + Dandelion (Plant + Ice) = Gummy or Fluorescent or Laser.

Special Notes:
Cool Fire can also be obtained through Laser (Fire + Electric) and Dandelion (Plant + Ice), Firebird and Ice.
Armadillo can also obtained through: medieval + alpine, pearl + alpine, jade+star, Zombie + Mud, and Pearl + flaming rock dragon
Gummy can also be obtained through: Firebird + fluorecent, firebird + star, Jade + star, and neon + nenufar
Pirate can also be bred from: Rattlesnake + lanternfish and neon + cloud

It should be additionally noted that Medieval + Alpine dragon is one of the best dragon breeding formula in Dragon City because of its ability to bred different types of hybrid rares.