I've been thinking about this type of mare since last year. The main thing that stops me from actually doing this is what would be a decent stat build to consider???
Str, int, hp, mana.
You get the idea. I figure I can grab a 2 slot mare, they are pretty easy to spawn. And give this a try
Is this for PvM or PvP? Necromare really shines in PvP and can be done with a Legacy nightmare, or a high 2-slot. With a non-legacy you will have to skimp more than a legacy, especially on Str and Hits, but that doesn't really matter for PvP. You don't need 600 HP in PvP (~400-450 is fine), and your pet will cap out on melee damage without the 700 Str. The power of the necromare in PvP comes with its spell burst potential (especially paired with Dismount).
If this is for PvM, the spec will not perform as well as Chiv, but can still be fun to mess around with. You
will have to make sacrifices somewhere, even on a Legacy mare. You will not be able to get full 120 scrolls, 700 Str, 20/10/30 Regens, 600 Hits etc.
To get a necromare to 700 Str, 20/10/30 Regens, 150 Dex/Stam, 365 Resists, and Full 120 Scrolls, costs between 4470-4620 points (out of 4503). 4470 is assuming the Nightmare is the
best possible legacy spawn with
max resists, 4620 would be a low-end legacy. That is without touching Hits, Mana, or adding a special move.
The most common place to cut from would be HPR, Magery/Necro scrolls, or Tactics scrolls. As SouthPaw mentioned, you want to naturally raise Dex/Stam to 125 first to save as many points as possible.