If I power the arduino by say 12 volt and 2A, is it possible to draw the equivalent power (~ 5A) (except for the power loss for the arduino itself) from the 5V pin?
The short answer is no, the long answer is maybe, but with a different setup.
The onboard regulator is linear so theoretically, ignoring any chip-specific limitations, you can't draw more current than you put in. 12V@2A in == 5V@2A out, the rest of the power 14watts is burned as heat in the linear regulator! This said, the regulators specced limit is 800mA, but thermal dissipation is going to keep you down in the 400-500mA range.
If you want to convert 12V@2A to 5V@4.5A or so, you'll need a 'switching buck regulator', and a beefy one at that. You're likely further ahead finding a cheap bench supply or repurposing a PC power supply.