Cheapest priced copy (Urza's Legacy): $16.99 at Card Kingdom, near mint, their price list as of .
Hidden Gibbons 
Enchantment · Rare · Urza's Legacy · #104
When an opponent casts an instant spell, if this permanent is an enchantment, it becomes a 4/4 Ape creature.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
1 printing · $16.99 to $84.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Urza's Legacy | 104 | Rare | $16.99 | $84.99 |
4 rulings from Wizards of the Coast
- 4 October 2004It changes into a creature even if the spell is countered.
- 4 October 2004When it turns into a creature, it is no longer an enchantment.
- 1 August 2008A 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.
- 1 August 2008The ability triggers when the spell is cast, so it becomes a creature before the spell resolves.