Unicorns, slowly developing a greater capacity for intelligence with their magical abilities, were likely the first of the ponies to start using tools. Their growing mental prowess, and the easier time they had taking down prey, had begun to take away the physical strength and stamina of their earth pony cousins, and they began to make up for it with sticks and stones. At first, swinging branches and throwing rocks was a way to do harm to hostile creatures, but as they grew smarter they began to sharpen their weapons.
Pegasi were soon to pick up on weapon use. Though they lacked the directed magic of the unicorns, flying allowed them to use their hooves to grasp objects, which they could then attack with. Earth ponies, having the least comfortable way of using weapons and the least need for them, likely took a fair bit longer to really start to adopt them. However, this lack of technological development likely did help to pave the way for their greater connection to nature and life.
Around the same time as weapons began to come into use, so likely did language. Ponies needed a way to convey ideas to one another, and so rudimentary words were formed to indicate certain objects and, eventually, concepts. As with tools, linguistics probably came best to unicorns, who, with their magic and tools, generally had more things to talk about. Pegasi, who could always see more than the two grounded races, also developed a wider vocabulary, while the earth ponies were stuck with very simple communication.
The Earth Ponies began to become outclassed by the other two species in matters of hunting. As they began to rely more on plants, they started to gain a greater understanding of them, and their language and tool use began to develop more towards agriculture. Eventually, they would gain their own lead compared to the pegasi and unicorns.
Thanks to the close proximity of the three races, none of them were ever too far behind the others in development, and though there were glaring differences between them, they were never quite enough to leave any of the three out of the joint species that is ponykind.
This is a bit of an experiment. I also don't know if I should try and tie this in with 'A Brief History of Equestria' or write my own separate ideas on the matter.
Also, apologies to writing so succinctly. I've always been one to either get to the point or unnecessarily embellish things, and I figure the former is superior.
Interesting for sure, keep going! The very short chapters and text book style make it an easy and quick read, great to drive those moments of boredom away, and so far, there are no contradictions that would have caught my eye (though I haven't thought about it too much yet). You've already begun pointing towards the change into herbivores, but I'm still curious how exactly you'll pull it off.