The pony types of Equestria an attempt to explain possible origins and the different origins of different pony types: Earth, Pegasi, and unicorns.

First let's start with the Earth ponies or ponies of the world of nature.  Their origin is a fairly simple one born of a fusion of the elements of the land they are able to use their magic to grow and alter the world of nature as they see fit at least with plant life as shown in G5. Why they didn't use it before could be any number of reasons perhaps they had lost their connection to such power or felt little need to use it beyond such a basic level. 

I suspect Granny Smith's "crazy rituals" may have been an attempt from her to bring back the stronger earth pony magics and skills with her family. Again all of this is speculation but at the very least I do highly believe earth ponies had the magic abilities long before G5. 

 Pegasi info now.  The pegasi hold two possible origins are that they are born of the element s of water and air AKA t the union of the sky and sea in holy unity. 

They possess the magic to alter the states of water and air from a simple calm breeze to a destructive thunderstorm and everything inbetween the ponies of the sky and sea hold reign over all forms of weather however they because their bodies require them to be very light weight they are unable to do any type of gardening or food growing as they do not possess the needed strength or connection with the earth to grow crops. 

You may be thinking but I've seen Dash, Lightening Dust etc cause MASSIVE destruction to property, and have strength seemingly equal to Applejack a nd yes that is true however their connections are to the SKY and SEA only NOT to the earth itself so if they attempted to plant they would be unable to grow anything just life if an earth pony were to attempt to control the weather they would not be able to do so correctly ether. 

Finally, we discuss the Unicorn race of ponies a very unique class of pony indeed as they born of mental beliefs and dreams of the other ponies their existence is a sort of paradox in that they were born of magic itself and yet since unicorns are the only race of ponies known to use magic it begs the question of which would have come forth first the power of magic that created the unicorn or did the unicorn appear first and learn to create magic?

Either way I suspect ultimately that the  two were for lack of a better "willed" into existence by either one or both of the other two pony classes perhaps this is why unicorns seem to have the ability to work with the earth and control weather through their magic it is done through their connection with their parents. 

The unicorns represent the power of the mind and the gift of co creation that is working with the power of the universe to build with it. When done correctly it can be used to bring to life the dreams and wishes of their fellow ponies and beings to bring happiness and love but when used incorrectly well... the Tantabus should give you some idea...

Earth, sky n sea and mind once mastered successfully will allow any pony that is not already an alicorn to become one they are a hidden fourth type and the watchers and protectors of the other three classes. It might even be possible that at some point in the future once mastered all ponies could very well become alicorns in which cases they would share and do the tasks together that each of the three classes had done solo. 

Unfortunately, without more info on the origins of the alicorns there is no way to know much more than this. 

I'm really not sure what else to say beyond though I would like to perhaps if I think of something else I will if you have questions feel free to ask and I will do my best to answer them.