Successes like Patriarch Poak aren't common; he had the lucky combination of a larger frame, a larger appetite, and the short, sturdy legs necessary to carry that weight around without his forelegs snapping in half.

For yinglets, it's like evolution decided "Hey, screw waiting around hundreds of millions of years to slowly develop gradual mutations and let the passage of time smooth them out... let's just do a whole lot of it at once!"

So basically, yinglets are what happens when nature's in too much of a rush to do the job in the usual way.

-Sal Valia