Loyal Pegasus can't attack or block alone.
2/1/2014 Loyal Pegasus can be declared as an attacker only if another creature is declared as an attacker at the same time. Similarly, Loyal Pegasus can be declared as a blocker only if another creature is declared as a blocker at the same time.
2/1/2014 If you control more than one Loyal Pegasus, they can both attack or block together, even if no other creatures attack or block.
2/1/2014 Although Loyal Pegasus can’t attack alone, other attacking creatures don’t have to attack the same player or planeswalker. For example, Loyal Pegasus could attack an opponent and another creature could attack a planeswalker that opponent controls.
2/1/2014 In a Two-Headed Giant game (or in another format using the shared team turns option), Loyal Pegasus can attack or block with a creature controlled by your teammate, even if no other creatures you control are attacking or blocking.