although you would need some sort of speed reduction gear train, a dentist's drill is air powered and quite small, and I have also seen air-powered "micro-grinders" the motor of which would fit into a 15mm X 50mm size envelope.

Pneumatic motor are often more efficient at high speed, and high speed allows to reach a good power to weight ratio which is needed for your motion application. Speed has to be reduced thank to mechanical gearbox.

http://www.mannesmann-demag.com/en/gruppen/25736.druckluftmotoren.html

How would you supply air to those robots? They are too small for holding tanks and will have less freedom with hoses (hoses may even turn they movement difficult, they could even be lifted from the ground).

Dear. Gustavo Kannenberg

Today, I'm using flexible rigid shaft for ratating gear in my robot and I hope to alternate rigid shaft to any other way. Therefore, I considered pneumatic line based rotating tools. In other words, I'm employing external generating motor. If I can utilize pnematic based power source such as air motor or drill, I will buy air-compressor for generating air pressure. As above researchers were written, I thought it based on grinders or drill at the garages. However, they are so bulk to me.

I hope that this answer helps to comprehend my question to you.

Thanks.

You will probably need a great amount of air to run something like a dentist drill. An air compressor would probably be greater than your robot.

Dear. Gustavo Kannenberg

Of course, I know that. But air compressor size isn't important to me, as it will be located far from robot. Also, general air-drill or motor require 7-9 bar according to investigation. Conventional air compressor can do that. 

Air activated systems (power supply) are "bulky, cumbersome, and respond to variations in temperature and humidity". I am not optimistic.