I imagine that's how it went. It was getting late, he was out of coffee/tea, and all he needed to do before wrapping things up was draw a dog. He doesn't bother digging out a reference book since, hey, it's just a dog. He starts sketching and fatigue makes his mind wander. By the time he comes back to his senses, the completed sketch of that abomination is on the page. After a brief look of horror, he laughs and calls it a night. He'll fix it in the morning.
...and then in the morning the inking phase starts and he starts on the dog and WHOOPS.
Either that or it's not meant to be any kind of dog. I mean, that thing can teleport/phase through solid objects...
I'm really thinking that Araki either was on a deadline to finish this or really didn't care about how the dog looked at that point. I mean he's made mistakes before right :V
Come on guys, that is obviously not meant to be a normal dog. Look at that thing. If that's a normal dog Araki needs to get his brain checked. My 5 year-old nephew draws better dogs that that.
And Part 2 > 3, of course.
I don't think Araki has brain problems it's just that it's one of the Araki things that he does. I'm sure that in the future the dog will turn full pony as Araki changes his mind