Hidden Herd

When an opponent plays a nonbasic land, if this permanent is an enchantment, it becomes a 3/3 Beast creature.

The List

Urza's Saga

When one of your opponents plays a nonbasic land, if Hidden Herd is an enchantment, Hidden Herd becomes a 3/3 creature that counts as a Beast.

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.

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.