When an opponent plays a nonbasic land, if Hidden Herd is an enchantment, Hidden Herd becomes a 3/3 Beast creature.
A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn’t matter how long the permanent has been a creature.
Does not trigger on lands put onto the battlefield by a spell or ability.
When it turns into a creature, it is no longer an enchantment.