Gollum's getting weirder. Some of these are good, some of these are bad. I'll let you tell me what's what.
I really like that Gollum isn't given much explanation in Riddles in the Dark. It makes it a lot stranger and a lot scarier. Bilbo just wanders further down into the mountain, having escaped the goblins, and finds this pool. The narration tells us that there are other things in the old mountains besides Goblins; things slithered their way in when the mountain was still young and stayed. After being nearly killed by the goblins, Bilbo finds something that eats those goblins there at the heart of the mountain. And on top of all that, it lives by some weird code and agrees to a riddle contest with Bilbo? So weird. Gollum is the strange midpoint between the dark unknown and the familiar.