I decided to repot this plant I've had for a couple of years in the same pot of soil for over 2 years. It was doing well outside in the summer...but over the last few months it got thinner and lost all of it's strange spikes (in the outdoor picture)

Does anyone know what it might be?

It looks like Austrocylindropuntia subulata 'Cristata' - Crested Eve's Needle. It lost its leaves because the leaves are only on new growth.

Crested Eve's Needle (Austrocylindropuntia subulata 'Cristata')

Interesting, thank you both!

I was worried it was some kind of euphorbia because my hands and arm I was holding it in are tingling. I'm guessing this must have small thorns or something, not sure what could be causing the irritation.

Anyways, hopefully it keeps growing. I have always liked its odd shape

Hi Brendan. Nice plant! The euphorbia has a very irritating sap that can give a bad rash or more to those sensitive to its toxicity.

I get a rash from quite a few of my non-Euphorbia cactus also.