The name "Necro" may refer to the mythic Necromancer, a conjurer of the dead, or the noun or prefix "Necro-" which means Death. This links in with the fact that it requires bones to make and the noise of the player sounding like a skeleton when hit.
The Necro armor in the previous updates (Before 1.0.6) used to increase walking/running speed. It maintained the same look, and could only be found in Jungle biomes.
There used to be a bug that the player's head was shown when the backtrail was active (patched).
It is possible that, due to the name and design, this could be a reference to Sauron, The Necromancer, from The Lord of the Rings Trilogy.
With the full Necro armor equipped in the Social slot, ranged attacks (such as Harpy's feathers) make the bone skeleton pain sound, but regular attacks make the human "Grunt".
When you use rocket boots with necro armor straight up on your way down your eyes will disappear for a moment