How could this pyramidal Mountain have been formed?
Such forms tend to be created by glacial activity, which, ahem, the ice-covered continent is known for. Much discussion of this in the related question in Skeptics: Are there three pyramids in Antarctica?

Here's the generic answer in Wikipedia:


  A pyramidal peak, sometimes in its most extreme form called a glacial
  horn, is an angular, sharply pointed mountain peak which results from
  the cirque erosion due to multiple glaciers diverging from a central
  point. It may be an example of a nunatak.


Here's a diagram of the formation process posted on Ace Geography:


Not knowing the specifics of the featured peak, as I understand it there are two general mechanisms of glacial erosion that apply:


Assume we begin with some sort of mounded terrain, or a relatively small protrusion on a larger continental surface like Antarctica. In a very cold environment, precipitation will fall as snow, mound up, and turn into ice. Generally, more ice will form on whatever side of the mound that is sheltered from sunlight (north slopes in the northern hemisphere, south slopes in the southern. Eventually the ice will move under its own weight, abrading the surface as it goes in a manner that will tend to exaggerate the original geometry of the mound. If the mound starts with or develops a slight dip in the slope, ice flowing off the back and side walls will dig a cirque or bowl. Depending on the starting conditions, your mound may develop several cirques; as ice grinds away at the lower elevations, you may wind up with a central, relatively slender peak with several ridges and valleys.
A second mechanism is having a glacier coming down a broader slope and cutting through an existing ridge. (Your ridgeline may begin with a triangular cross-section, or may have developed one from the glacial processes mentioned above.) At some point, ice from a higher elevation will flow across or through your ridgeline (just as a river would), eroding a gorge through the ridge. 


Inevitably, given enough material to work from, and even with a relatively smooth starting surface, the erosive power of ice will give you steep slopes and some pyramidal structures.