I have a somewhat unusual background, in that I'm an adopted kid. Not that being adopted is particularly unusual, but that I was adopted twice. First time I was 6 years old and then I was put up for adoption again at age 9.
When I first learned about the platypus all those years ago, I was quite taken with it; while others might pick wolves, eagles, or bears as their totem animals, the platypus always seemed far more appropriate for me. And no, I'm not an Aussie either.
Fast forward to 16 years ago and I'm starting to get into LAN parties and doing a lot of online computer games. After some careful thought, I settled on Scurvy Platypus as my primary identity online. It had some symbolic meaning for me (Platypus), I always thought "scurvy" was one of the happier sounding illnesses, and the combination was unusual enough that I'd likely never have to worry about "sharing" an identity with anyone else. And it sounded kinda playful too.
I've kept it ever since, across games and message boards. If someone thinks I'm groovy, it shouldn't be too hard for 'em to remember me/keep in contact; for those that dislike me, it shouldn't be too hard to avoid me either.
Funny thing about my name these days, is I moved to New Zealand in 2009. So I get a chunk of folks that are confused and think there's platypuses in NZ (there isn't), the usual folks that assume I'm an aussie (no aussie blood that I'm aware of), and a chunk of Kiwis that figure I'm a stupid/confused american.