Creeping Tar Pit
Land
This land enters tapped.
: Add 
or 
.
: This land becomes a 3/2 blue and black Elemental creature until end of turn and can't be blocked this turn. It's still a land.
Jason Felix
Rulings
When a land becomes a creature, that doesn't count as having a creature enter. The permanent was already on the battlefield; it only changed its types. Abilities that trigger whenever a creature enters won't trigger.
(wotc • 2010-03-01)
A land that becomes a creature may be affected by "summoning sickness." You can't attack with it or use any of its {T} abilities (including its mana abilities) unless it began your most recent turn on the battlefield under your control. Note that summoning sickness cares about when that permanent came under your control, not when it became a creature.
(wotc • 2010-03-01)
Rulings
When a land becomes a creature, that doesn't count as having a creature enter. The permanent was already on the battlefield; it only changed its types. Abilities that trigger whenever a creature enters won't trigger.
(wotc • 2010-03-01)
A land that becomes a creature may be affected by "summoning sickness." You can't attack with it or use any of its {T} abilities (including its mana abilities) unless it began your most recent turn on the battlefield under your control. Note that summoning sickness cares about when that permanent came under your control, not when it became a creature.
(wotc • 2010-03-01)