The rules aren't so clear:
PHB pg 291:
When you ready an action, you prepare to react to a
creature’s action or an event.
Choose the specific action you are readying (what attack you plan to
use, for example) as well as your intended target.
It say to choose the target, but in Dungeon Master's Kit DM's book pg 160 the rule is slightly different:
Choose the specific action the creature is readying (the attack it plans to use, for example), as well as the intended target, if applicable.
The target is become almost optional.
Also look the example (in either sources):
As soon as the troll walks
out from behind the corner...
If the troll is behind the corner you don't have line of sight to him, right?
You know that a troll is there but you don't see him.
Don't forget that in D&D4° the players have many informations so when the encounter begins the DM probabily places all the monsters on the board and the player know about them and he can acts in conseguence, even if they are out of sight for the PC.
BTW 'the creature who walks out of the corner' IS a target and is specific. Nowhere say to name it.
Finally you can ready an action to react to a creature’s action or to an event. You RAW can consider 'When a creature becomes a valid target' like an event and ready an action against him.