It has 3-4 original units and the rest of the units it uses r infected versions of terran or protos units,note that the zerg cannot be infected as they seem to have an immunity to it.
The Beast is an enitity which seeks to "Feed"(thats infect other life forms) on "Parts"(life form in question) and to grow.It not only gains more mass by "feeding" but somehow it is also able to asimilate the knowledge,feelings and so on of the creature it just infected.This makes the Beast a very deadly foe as it can quickly grow and asimilate knowledge which it uses to augment its forces.Note that the beast its self is a huge bio-computer and uses organics and conductive metals to form new circuits.The Beast also has a main leading entity but if this hapens to die another will take its place.
If i have to place the Beast in the gradint sistem used by the Xel'Naga,it has the purity of body(every part of the Beast has a specified role) and the purity of tought(it knows what it wants and it will not stop until it gets it).