Swamp Mosquito | Magic: The Gathering
Formats Swamp Mosquito is Legal in
Official Oracle Text for Swamp Mosquito
Whenever Swamp Mosquito attacks and isn't blocked, defending player gets a poison counter. (A player with ten or more poison counters loses the game.)
Official Rulings for Swamp Mosquito
4/15/2013 : An ability that triggers when something "attacks and isn't blocked" triggers in the declare blockers step after blockers are declared if (1) that creature is attacking and (2) no creatures are declared to block it. It will trigger even if that creature was put onto the battlefield attacking rather than having been declared as an attacker in the declare attackers step.