We have the same type of problem here too --spiders. DS is both fascinated and scared of them.
Two months ago, my SIL's family made a few not so smart comments to DS when I said that he doesn't have nightmares. They asked him if he had monsters in his closet. Gee thanks for planting ideas. So then we had the start of our problems.
Got through the monsters in his closet. I threw cookie monster, Sully and Mike (monsters inc), and elmo into his closet one morning. That night when he asked if there were monsters in his closet I said," yes you do!". He was a bit puzzled and so I opened the closet door and showed him. He saw his 'buddies' and smiled. Gave them all a big hug and wanted them back into his closet because they like to sleep there. No more monster problems since.
Spider problem. This one is more on going thanks to Halloween. He saw a rather large spider at my parents' house and got scared. Tried to personify them a bit, saying they like their own house, that one got lost on his way home and ended up inside the house. Worked for a few nights. Recently, we made signs 'no!' with drawings of cute spiders that he helped with. We put them on the front door and on his bedroom door. Seems to be working so far. The next thing I'm looking at doing is finding kids' story books on spiders.
Hope there are some ideas you can use.