About 2 years ago we had "Monkeys". D1 was about 2.5 years old at the time and it was a total nightmare. She was terrified and would leap into my arms trembling at the slightest noise. They stayed for about 2 months and were finally banished following trips to various zoos and lots of nice monkey stories.
D1 is now 4 and a half and this time we have monsters. I have a permanent shadow. The fear doesn't always seem quite as genuine as it was with the monkeys and I feel some of it is for attention as she only has to mention "monsters" and Mummy jumps into reassurance mode. I am not saying by any means that it is a total fabrication, just that she is a smart cookie and with me being pregnant (I was pregnant last time too) it is an effective way of getting some one on one attention.
I do have to stifle a little giggle when she will not go downstairs by herself if the rest of the family are upstairs without taking D2 with her. D2 is 20 months. Does she really think that if there was a monster, a 20 month old would protect her?
At the moment it isn't too much of a problem as it is not affecting bed times. We do have the odd wakening throughout the night but she is usually very calm when telling us she can hear monsters, possibly just looking for the green light to climb into bed with us for the night. I am, for the time being, fairly hopeful the monsters will vanish before they become too much of a problem.