Yaks, and anything with the [GRAZER] tag require a pasture to be defined on edible grass (cavern moss and similar plants work as well) and them placed on it or else they'll starve to death, they can't eat grown plants that your dwarves eat. I don't bother with grazers, I just slaughter them the moment the migrants bring them.
Necromancers are BASTARDS. If you reclaim a fort with 80+ dwarves and a large refuse pile stocked with dead goblins, god help you if the fort was near a tower. The necromancer(s) will come in an ambush (GRAAAAAAH) and raise everything outside. You'd best be self-sufficient and have a drawbridge or a hell of a lot of weapon traps. Generally, I just lock up everyone in the fortress and raise the gate then wait for goblin ambushes to pop up next to the necromancers and slaughter them. Due to the lack of precision targeting for soldiers, you've no hope of ever killing a necromancer with at least 5 zombies around them.
They make a great defense system if you set up a rig that gives them sight to a room full of bodies, which they raise, then release the zombies when goblins come and make the necromancer not see any new bodies in the room (usually via drawbridge or floodgates in front of some fortifications).
The wiki claims they "sometimes" take apprentices. This is complete bullstuff. I've found 20 necromancers in one tower after 550 years. To make them better, it seems that the original necromancers just love having their secrets to raping nature on some serious bling, like solid platinum slabs. Drop it from a z-level up and watch anything below explode in a pile of gore.
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I'm trying to gen a world that goes on for 10000 years. Max population = 100000 (as high as it could go) world size = small (65x65). The first run, I set it so that 100% of megabeasts needed to die before worldgen stopped. 147 years in, all of them died and the world hit The Golden Age. Bah. Second time (now) it's at year 1443 and it's in the Age of Myth. Worldgen started with 40 megabeasts and 50 semimegabeasts.
EDIT: Vampires are technically dead. Dead things ignore each other, but a hostile necromancer will still be hostile to one of your dwarven vampires. Cheap, effective. Have fun.