Probably Draenei Children.
Probably Draenei Children.
Monkey skulls? They look really small.
The other skulls in the background appear human, could have just been collected after the portal was re-opened.
Blizzards loves skulls. And yes, I see the validity of your point, but the fact is, Blizzard has made a lot of bigger mistakes over the years. I strongly believe more epic mistakes are on the way.
Maybe Sporelings? They're small enough. Not really sure if their fin is bone, or if they even have bones for that matter...
Regardless, it looks like a mix of several species, but oddly small.
Depends on if their horns are actual bone or not.
Given the wide diversity of horn shapes and sizes, as well as the presence of draenei who don't have horns, they could be something else.
Plus we're talking about a race with cloven hooves, digitigrade legs, tails and face tentacles. Draenei are weird.
Human and orc skulls are vaguely similar except for the extra orcish features on their skulls.
Look at a child orc. In their infancy or childhood their face structure is exactly the same as a human; their large jaws grow out as they mature. They're likely undeveloped orcs which would have the same skull shape as the ones on Gul'dan.
He's already playing with demonic magic, maybe it's one of these guys:
Mother pus bucket!
They are the skulls of Sand Gnomes:
http://wowpedia.org/Sand_gnome
This race of gnomes are from Draenor living at least in Terrokar Forest but may be found all over the continent.
I'm just thinking it's orcish baby skulls, because evil warlock and stuff.
*shrugs*