DS8 had an imaginary cousin a couple of years ago, which is one of the few things he's ever shown any imagination about. In his own words, he's "not a fiction kind of guy" for the most part, so I just let him go with his cousin when he had him. I haven't heard anything about Cousin George for a long time now.
Oddly enough, DD5 hasn't mentioned any particular imaginary friends, and she's the one who has all the imagination in the family! She does all kinds of imaginary play with her stuffed animals, though. She probably has so many critters that she doesn't need to make any up!
