The color and scent of their fur changes to match the mountain grass. When they sense hostility, they hide in the grass.
In Pokémon Black 2 & White 2, the researcher in the Season Research Lab along Route 6 would give the player a Deerling with it's hidden ability.
The form of the Deerling depends on the season it was received on.
(In PKHeX, setting the event flag 0426 allows players to re-receive this gift)