Hello all,
Because I am of the opinion that adding two more materials to the abundance of special materials (adamantine, alchemical silver, cold iron, mithral), is redundant and just confusing for new players, I decided to come up with a way to replace DR/byeshk and DR/targath with probably either DR/cold iron or DR/silver. Especially considering that cold iron etcetera have their roots in real world folklore, as opposed to byeshk and targath, which are fictional altogether.
Since I don't know what to replace DR/byeshk and DR/targath with, I turn to this forum's help. Many thanks in advance. 
Anyways, here's what I could find in the MM and ECS:
DR/cold iron: bodak, some demons (CE outsiders), dryad, all eladrins (CG outsiders), night hag, nymph, satyr, all sprites, dusk hag* [basically all fey, some chaotic aligned outsiders, some hags and bodaks]
DR/silver: some devils (LE outsiders), all guardinals (NG outsiders), all lycanthropes, all nightshades, vampire, vampire spawn, yeth hound [basically lycanthropes, vampires(pawn) and various non-chaotic aligned outsiders]
DR/byeshk: daelkyr*, dolgaunt*, dolgrim*, mindflayer* [basically all the natives from Xoriat]
DR/targath: ascendant councilor*, undying councilor*, undying soldier* [basically all undying court deathless]
*ECS only
So what do you think would best to replace byesk respectively targath DR with?
Maybe the folkore of (cold) iron and silver can help out here as well: "[cold] iron could ward off ghosts, fairies, witches, and/or other malevolent supernatural creatures." || "Most modern fiction describes werewolves as vulnerable to silver weapons" [Wikipedia, 2009]
I would probably go with Cold Iron for deathless (As elves are kinda-sorta fey in 3.5, and actually are in 4e).
For the Daelkyr, I think I'd actually go with Adamantine, like for constructs. Not necessarily because it's so hard, or because they're so tough, but because it's the strongest naturally occurring metal, and is therefore the antithesis of the influence of Xoriat.
Or you can go with Silver, because it represents "purity", which could also be considered diametrically opposed to the corruption of the Far Realm.
I would do it the other way around. Pretend that Cold Iron doesn't exist, and instead replace it with Byeshk, merging the two. The downside is that I think the dwarves mine cold iron in the east. If any of special materials get barely cursory mention in the ECS, replace it with byeshk and merge. Cold Iron looks like about the best option though considering it seems to focus on outsiders, and the byeshk-folk are somewhat native abominations created by outsiders. Also, purple is a cool color.