The Tarbagan Marmot is an endangered species in Northern and Western Mongolia (in sections of the Mongolian steppe). The closest letter in H.I.P.P.C.O to suit the reason for this animal's population decrease is "O", for over harvesting. The Tarbagan Marmot is endangered due to them being over-hunted. They're made into a traditional dish called "boodog" that's been a traditional dish for centuries.
Banded Elm Bark Beetle
There aren't like any really major invasive species in Mongolia, probably because it's not exactly a place of heavy traffic flow going in and out. One of the only TWO I could find is the Banded Elm Bark Beetle (Scolytus schevyrewi), which is classified as an invasive species and a pest, likely consuming plant life in significant quantities. They were most likely introduced on accident, because there's no real benefit of them being there that would motivate someone to make the effort to introduce them intentionally.
More commonly referred to as the Mongolian Oak, this tree is prevalent in not only Mongolia, but China, Japan, Korea, amongst other Eastern Asian nations. It's found in drier environments, which is why the Mongolian steppe is the ideal place for it to be, with its lower annual rainfall.