Sand Damage and Rain Dish

[Medium Kevin]

a lotad is on the bench and has no damage on it and the opponents dark tyranitar is active. which body goes into effect first? sand damage or rain dish?

rain dish: at any time between turns, remove 1 damage counter from lotad.

 

[PokePop]

Per rulings from TPC, in general, the owner of the Pokemon being affected by multiple effects at the same time gets to choose the order. This includes those effects that occur "at any time between turns".
So, you could choose to place the damage counter and then remove it one turn (if your Lotad started with no damage on it) and on a later turn remove a counter and then place one (to protect a Lotad on the verge of being KO'd).