Dry Bone Skeletons
 

These were invulnerable to melee(via non-magic weapon) from my Druid in Oasis of Marr. The only undead type that I recall being invulnerable to melee were the ghouls. They weren't invulnerable in classic and I don't think they are even invulnerable in EQ now. (Nothing indicates that they are now invulnerable anyhoo)

I double checked some posts and nothing shows Dry Bone Skeletons being invulnerable to melee but posts do support ghouls being invulnerable without a magical weapon.

The following is some support since the pet cannot damage magically invunlerable enemies without the proper weapons and this strategy primarily relies on the damage from his pet it seems. I doubt people would hand out magic items to a pet just to hunt these especially since they were much more harder to come by in classic.

http://eqbeastiary.allakhazam.com/search.shtml?id=2468

(2000)
"If you are a necro level 16 or higher, there is an easy, mana efficient way to beat these down:
Engulfing Darkness (60 mana)
Spook the Dead (10! mana)
Pet beats on it.
Refresh Spook every ten seconds or so to make sure it doesn't wear off (it WILL be resisted some of the time)
Refresh Darkness as necessary.
Stay a little bit away so it can't jump you immediately if it does break Spook.
Oh, and of course, make sure you know which way Spook sends it, because you don't want to run into another one :)"

(In support of ghoul being invulnerable)
http://eqbeastiary.allakhazam.com/search.shtml?id=140

(2002)
"You must have a weapon that says "MAGIC ITEM" in its properties in order to hit ghouls."

Fixed, pending update

Ice Boned Skeletons in Everfrost also required magic weapons. Those things were the bane of teens-level players trying to hunt out in the tundra. I can't recall fighting them on P1999 to be able to say one way or the other how they're flagged here.

I recollect Dry Bone Skeletons as annoying mostly because they were sometimes hard to see in the dunes and tough for their level due to the fire special.

Danth