The ghostly whelpling minipet uses the same model as the crimson whelpling, instead of a ghostly dragon whelp model as its name would suggest. It’s not just a missing “ghost” effect on the pet - It also uses the crusty old vanilla-era dragon whelpling model rather than the modern one introduced in legion.
Probable placeholder that was never gone over and corrected for release to live.
I think the goal was that it was a Crimson whelp that died along with all the other red dragons on Crestfall during the battle. It should have a ghost effect on it, if it doesn’t that certainly would be a bug.
Aha, it actually does have the ghostly effect, but the pet has to be summoned to see it; it is not seen on the pet journal. Unlike the drowned hatchling where the ghostly effect is baked into the pet’s model. Strange implementation suggesting intern work or just laziness.
So I suppose that part of it is fine, though the pet still uses the hideous and outdated vanilla dragonling model instead of the high-def one introduced in legion. There’s no reason to use outdated assets for new pets/mounts/whatever - kindly fix this mistake and use the modern asset.
As of October 9, Ghostly Whelpling is definitely showing the wrong appearance when summoned, is looking exactly like the Crimson Whelpling instead of ghostly gray color.
10/14 - My ghostly whelpling is not so ghostly…he has reincarnated as a crimson whelpling. Color me disappointed. 
Ghostly Whelpling is Ghostly during Pet Battles, but when summoned as a companion he’s still the Crimson Whelpling colour.
Can confirm. Ghostly Whelpling is only ‘ghostly’ when in a pet battle. In the pet battle menu and when summoned, it looks like the old school Crimson.
Oh, look. This is still bugged, a year later. Wish I would have known before blowing all of my gold on a bugged skin.
Necroing this post, because, it’s still bugged. Blizzard, please fix…