That is crazy;)! I wonder what type of environment it was in for that cactus to grow like that. Honestly I wouldn't do anything with this cactus till spring. Keep it dry till March. Once Mid March comes around I would repot. Wait I'm assuming you live on the Northern Hemisphere. Could you give your hardiness zone too. That helps when giving advise;).

When you repot you can use the same pot, but if the roots are week, which I suspect they are, go down a size in the pot size. Don't go up:). Use a fast draining mix. Bagged cactus mixes almost always need to be amended with some perlite and or pumice. You may want to look up something called Al's gritty mix.

I'm so very curious about the roots on this guy. Just wait till spring though.

This guy is difficult to id because of the shape. I'm pretty sure he wasn't receiving proper light at one point. That's why it has that hourglass shape. Maybe some type of Echinopsis:). Very strange how the cactus has ribs at the bottom, but it's starting to get tubercles at top. You may want to think about starting this guy over next growing season and cutting him, but hey this guy is different. May be neat to see what he grows out looking like.

Hi!I'm not much of an expert so feel free to correct me if I'm wrong, but pretty sure that looks like corking. It's just something that happens to cacti as they age. I have a few cacti with a similar appearance and they're perfectly healthy, just woody at their bases.

Hope this helps