Terrain Walk

The monster ignores difficult terrain in encounter areas where the stated environment predominates.

Earth Walk: The creature ignores difficult terrain if that terrain is the result of rubble, uneven stone, or earth.

Forest Walk: The creature ignores difficult terrain if that terrain is the result of trees, underbrush, plants, or natural growth.

Ice Walk: The creature ignores difficult terrain if that terrain is the result of ice or snow.

Swamp Walk: The creature ignores difficult terrain if that terrain is the result of bog, mud, or shallow water.

Published in Monster Manual, page(s) 283.