Sleeping enchantment

Sleeping enchantments are enchantments that do nothing ("sleep") until triggered ("wake"). "Sleeping" enchantments were printed in the Urza's block. Each sleeping enchantment that required your opponent to cast a spell originally read "successfully casts", these were errataed to read just "casts" so they work even if the spell is countered. When sleeping enchantments turn into creatures they are no longer enchantments.

Rulings

 * Most of these enchantments have a triggered ability that, when it resolves, causes the enchantment to become a creature. Some have an activated ability that does that. The ability specifies the power, toughness, creature type(s), and additional abilities of the creature.
 * Once the ability resolves, the permanent is no longer an enchantment. The card type "creature" overwrites the card type "enchantment."
 * Everything else about the permanent remains the same. For example, Opal Gargoyle turns into a white creature that's still named Opal Gargoyle. Its mana cost is still . If it has any counters on it, or has been affected by any spells or abilities, they continue to affect it.
 * The ability's effect has no duration. The permanent remains a creature until the game ends, it leaves the battlefield, or some other effect causes it to become another card type. Similarly, it retains each of its new characteristics until the game ends, it leaves the battlefield, it ceases to be a creature, or some other effect changes any or all of those characteristics.
 * When an enchantment becomes a creature, that doesn't count as having a creature enter the battlefield. The permanent was already on the battlefield; it only changed its types. Abilities that trigger whenever a creature enters the battlefield won't trigger.
 * A "sleeping enchantment" that turns into a creature may be affected by the "summoning sickness" rule: It can attack, and its activated abilities with undefined in the cost can be activated, only if its controller has continuously controlled that permanent since the beginning of his or her most recent turn. Note that summoning sickness cares about when that permanent came under that player's control, not when it became a creature.
 * Some "sleeping enchantments" have abilities that trigger when an opponent casts a spell of a certain kind. For example, Opal Gargoyle's ability triggers when an opponent casts a creature spell. If an opponent casts a spell that meets the criteria, these abilities trigger and resolve (causing the "sleeping enchantment" to become a creature) before that spell resolves.
 * Some "sleeping enchantments" have abilities that trigger when an opponent plays a land of a certain kind. If an opponent plays a land that meets the criteria, these abilities trigger and resolve (causing the "sleeping enchantment" to become a creature) after the land enters the battlefield. If that land has an "enters the battlefield" ability with a target, the "sleeping enchantment" will not yet be a creature at the time the target is chosen.
 * Each triggered ability that animates a "sleeping enchantment" has an "intervening 'if' clause" that checks to see if it's still an enchantment. That means (1) the ability won't trigger at all unless the permanent is still an enchantment at the time the trigger event happens, and (2) the ability will do nothing if the permanent has somehow ceased to be an enchantment by the time the ability resolves. (So even though each of these permanents retains its triggered ability even after becoming a creature, they won't trigger again while those permanents are creatures.)

Black

 * Lurking Evil Pay 1/2 your life, rounded up / becomes a 4/4 Horror creature with flying.

Blue

 * Veil of Birds When opponent plays a spell / becomes a 1/1 Bird creature with flying.
 * Veiled Apparition When opponent plays a spell / becomes a 3/3 Illusion creature with an upkeep of  (or sacrifice it).
 * Veiled Crocodile When a player has no cards in hand / becomes a 4/4 Crocodile creature.
 * Veiled Sentry When opponent plays a spell / becomes an Illusion creature with power/toughness equal to the total casting cost of that spell.
 * Veiled Serpent</c> When opponent plays a spell / becomes a 4/4 Serpent creature that cannot attack unless defending player controls an island. This is the only sleeping enchantment with cycling.

Green

 * Hidden Ancients</c> When opponent plays an enchantment spell / becomes a 5/5 Treefolk creature.
 * Hidden Guerrillas</c> When opponent plays an artifact spell / becomes a 5/3 Soldier creature with trample.
 * Hidden Herd</c> When opponent plays a nonbasic land / becomes a 3/3 Beast creature.
 * Hidden Predators</c> When opponent controls a creature with power 4 or greater / bcomes a 4/4 Beast creature.
 * Hidden Spider</c> When opponent plays a creature with flying / becomes a 3/5 Spider creature with reach.
 * Hidden Stag</c> When opponent plays a land / becomes a 3/2 Beast creature. Hidden Stag turns back into an enchantment; if you play a land.

White

 * Opal Acrolith</c> When opponent plays a creature spell / becomes a 2/4 Guardian creature. You may pay  to turn back into an enchantment.
 * Opal Archangel</c> When opponent plays a creature spell / becomes a 5/5 Angel creature with flying and vigilance.
 * Opal Caryatid</c> When opponent plays a creature spell / becomes a 2/2 Soldier creature.
 * Opal Gargoyle</c> When opponent plays a creature spell / becomes a 2/2 Gargoyle creature with flying.
 * Opal Titan</c> When opponent plays a creature spell / becomes a 4/4 Giant creature with protection from each of that spell's colors.

Black

 * Lurking Skirge</c> When a creature is put into one of your opponents' graveyards / becomes a 3/2 Imp creature with flying.

Green

 * Hidden Gibbons</c> When opponent plays an instant spell / becomes a 4/4 Ape creature.

White

 * Opal Avenger</c> When you have 10 life or less / becomes a 3/5 Guardian creature.
 * Opal Champion</c> When opponent plays a creature spell / becomes a 3/3 Knight creature with first strike.

Black

 * <c>Lurking Jackals</c> When an opponent has 10 life or less / becomes a 3/2 Hound creature.

From Time Spiral
In an homage to things of the past, Time Spiral included one sleeping enchantment.

White

 * <c>Opal Guardian</c> When opponent plays a creature spell / becomes a 3/4 Gargoyle creature with flying and protection from red. It is an homage to <c>Abbey Gargoyles</c> from Homelands.