Yes, when a target makes an attack against a creature, the creature can be the target itself.
Assuming the power it is using to make that attack allows the creature to be the target of its own power, not all powers do.
Ahhh but...Powers like Drunken Monkey become very difficult to use in an environment where a creature is not allowed to attack itself. Therefore, I would not recommend house ruling to disallow creatures to attack themselves.
...you cause one of your foes to attack its companion.
Can you give an example? I understand a power that targets an "enemy" would not be able to target yourself (like a wizard could not target himself with Winged Horde). But any attack that targets a "creature" should be able to target yourself, no?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.