Enchantment, G (1)
Rare
When an opponent plays a nonbasic land, if Hidden Herd is an enchantment, Hidden Herd becomes a 3/3 Beast creature.
Illus. Andrew Robinson

10/4/2004: When it turns into a creature, it is no longer an enchantment.
10/4/2004: Does not trigger on lands put onto the battlefield by a spell or ability.
8/1/2008: A noncreature permanent that turns into a creature is subject to the "summoning sickness" rule: It can only attack, and its {T} abilities can only be activated, if its controller has continuously controlled that permanent since the beginning of his or her most recent turn.
Legal in Vintage (Type 1)
Legal in Legacy (Type 1.5)
Legal in Classic (MTGO)
Legal in Commander