I haven't used RPGMaker XP, but I can't imagine it's much different than 2000 or 2003.

Set a switch, let's say 10, to OFF. Then set up switch 10 so that when it is ON it plays an animation of the monster running towards you. Then in the game, modify the door opening/changing maps event so that switch 10 is turned on. This will make the monster run towards you. Then, after the monster runs towards you, turn off switch 10 in the "monster running towards you event" so that it doesn't continuously repeat.