Hey, guess the Pokemon with the clues provided. Tip: don't think the game way. Think outside the box.
Earthquakes can't harm him, but Hitmonlee makes him weep,
As long as there is water in air, he jollies,
But when there's sand, he cries,
So terrified of Groudon, so red he becomes no matter how much he tries....
Earthquakes can't harm him because he is floating all the time, Hitmonlee makes him weep because normal is weak against fighting
when there is rain, he jollies by changing its form
during sandstorm it cries, because he can't change form and gets sandstorm damage
Groundon causes drought, so Castform changes into sun form and hence becomes red.