The short tank is called Wombat.
It was just a tiny, mobile mockup. I think it was powered by a sedan engine or something. The US Army's TARDEC/TACOM (Tank and Automotive Research, Development, Engineering Command/Tank and Automotive COMmand) basically just built it in a garage to prove that they could theoretically make a tank tinier than T-80/T-72/T-64.
edit: It was also called HD-40T, presumably the "40T" meant 40 tons. I think it was part of the Future Combat System (singular) which was a TARDEC study looking at what a smaller, lighter M1 would look like. That whole study and associated mockup was done between '93 and '95. It vanished when they realized there was no tangible difference in mobility between 40 tons and 55 tons I guess. Then Future Combat Systems (plural) starts and they decide to try to make a 20 tons thing to replace M1 instead.
The TARDEC official history is sort of vague on Wombat, basically just mentioning a Discovery Channel show it was in and a picture of thing, because it's mostly an awkward looking footnote.
The weird rhombus barrel shroud is probably to get rid of radar reflections caused by ordinary round barrels; at the time, everyone and their dog was sort of obsessed with copying stealth fighters.
tl;dr: It was a mockup of a futuristic tank to replace M1. Then the US Army decided to make FCS Manned Ground Vehicle instead.
edit2: Muv-Luv is based.
