Cheapest priced copy (Urza's Saga): $0.35 at Card Kingdom, near mint, their price list as of .
Hidden Spider 
Enchantment · Common · Urza's Saga · #264
When an opponent casts a creature spell with flying, if this permanent is an enchantment, it becomes a 3/5 Spider creature with reach. (It can block creatures with flying.)
Legal inCommanderLegacyVintagePauper
Not legal inBrawlStandardPioneerModern
1 printing · $0.35 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Urza's Saga | 264 | Common | $0.35 | — |
4 rulings from Wizards of the Coast
- 4 October 2004When it turns into a creature, it is no longer an enchantment.
- 1 April 2008This card now uses the Reach keyword ability to enable the blocking of flying creatures. This works because a creature with flying can only be blocked by creatures with flying or reach.
- 1 May 2008The ability only triggers if the creature spell has flying, regardless of whether an effect would cause the creature to have flying once it entered.
- 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.