Spawning Pool
Land
Spawning Pool enters the battlefield tapped.
: Add 
.
: Spawning Pool becomes a 1/1 black Skeleton creature with ": Regenerate this creature" until end of turn. It's still a land. (If it regenerates, the next time it would be destroyed this turn, it isn't. Instead tap it, remove all damage from it, and remove it from combat.)
Illustrated by
World Championship Decks 1999#js142UncommonEnglishLegality
Rulings
A noncreature permanent that turns into a creature can attack, and its 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.
Activating the ability that turns it into a creature while it’s already a creature will override any effects that set its power and/or toughness to a specific number. However, any effect that raises or lowers power and/or toughness (such as the effect created by Giant Growth, Glorious Anthem, or a +1/+1 counter) will continue to apply.