It sounds good, but those ponies though..
As he pointed out with his notes, these ain't normal ponies. These are ones forced to live not just by
wasteland, but
dwarf fortress rules. They have as much mercy to spare the world as dwarves do - that is to say, virtually none.
If you want just a general difficulty spike, there's also
Agricultural Revolution, which greatly decreases farming and herbalism's outputs, sans for trees which will possibly form the basis of your food industry in forested areas, and make others far harsher to survive in. There's also
Lands of Duality which make Good and Evil biomes equally horrible in their own ways, and
What Lurks Below, which makes the caverns even more dangerous than they already are. There's also
Zombies Ate My Dwarves, which adds a number of exciting new monsters with a B-Movie Sci-Fi/Horror flavor to evil biomes and caverns as well as various hostile civs such as land pirates, martians, and hillbilly cannibals. This last one recently got a nice DOOM flavored update recently.
Fortress Defense, which Telgin recommended, adds tons of new civs to fight from weak Dark Strangler hordes to steel-equipped Greater Badgermen, and
Grimlocke's mod changes various things with equipment and combat, and optionally makes iron and steelworking more realistic/difficult. There's also my own
prehistoric creature pack, which while nothing expansive, adds many animals to be seen outside savage biomes, some of which like the aurochs and great elk might randomly decide they don't like your dwarves, or can be encountered in both mountains and caverns - consequently making mountains a bit more dangerous, due the the presence of Cave Lions and Bears on the slopes as well as a variant of terror bird.
My own Disclaimer: Never used Grimlocke's. I've always used Stal's Armory when I wanted better weapons and armor related stuff.