Toddler from hell may be a little harsh. Plus he's 3 1/2 so he really isn't a toddler. But I'm not sure where our surprise caboose baby fits into the grand scheme of things.
This kid is smart. Smarter than all of his siblings. And the funny thing is his siblings are have gifted level IQs. He blows them all out of the water. Nothing gets past him and he will call you on everything. He also thinks everything is up for negotiation.
Example: Toddler from hell is tucked into bed. 5 minutes later toddler from hell slips downstairs and is sitting on the sofa because he's "not tired". Toddler is told that it doesn't matter that he's not tired, it's bed time and he needs to get into bed. Toddler responds that he will not be going to bed. Toddler is told that if he doesn't get his butt into bed immediately, he will be carried upstairs and put into bed and lose his favorite toy for the next day. Toddler then informs his mother that he no longer loves her because she makes him do things he doesn't want to.
So. I think he will officially be known as The Negotiator. Imagine William Shatner in the priceline commercials, but as a cute lucid 3 year old.