It's all speculation - but based on the viral marketing campaign and the ARG game JJ Abrams created there is a lot of speculation on what MPG (Cloverfield) actually is.
Rob was going to Japan to market a product called "Slusho". His brother in film wears a Slusho shirt. There is a lot of speculation that drink was somehow related to ingredients obtained from this Monster. The company that owned Slusho, Tagruato built a oil mining rig off the East coast that was believed to be "mining Slusho" ingredients.
http://www.floridaventureblog.com/2008/01/cloverfield-building-arg-climax.html
Two weeks ago that platform was destroyed - by what is believed to be the MPG (Cloverfield)
There is speculation that this monster either was dormant and awoke and is of this earth. There was also a Tagruato satellite that crashed about a month age in the Atlantic. That may be what is captured in the background when they were filming in April at Coney Island.
That potentially brings Alien into play. Also writer recently said the monster was a baby, lost, confused and angry.
The side effects of taking too much Slusho is immediate happiness and then a crash to very violent aggressive behaviour. Also Tagruato was sending a tanker from rig area to NYC. It is pretty clear from movie that explosion was MPG destroying an oil tanker and it exploding. New also reported a oil tanker submerged.
Till sequel we won't know. I think it is alien.