What is this!!!!??

Hello Everyone,
As I was getting gas the other day, I noticed there was an antifreeze leak and noticed this wacky thing on one of the coolant lines where coolant was dripping out. Even more, it's capped literally with a soda cap. I just bought the car a few months ago and never even noticed this under the hood. Check it out and fill me in as to why the previous owner might have installed this.

Also, what type of rear end is this? Thanks!!!