Definitely not the best, but the best ones don’t fly sadly.
I have been using the black cloud serpents too. Not exactly DK, but they are black and aren’t oversized. I do have the sha weapon illusion which does look great with the heavenly onyx cloud serpent.
Cant get the plagued proto drake anymore either.
There are a lot of death themed mounts. You can get the Blue Proto-drake. There are the maldraxxas mounts, Fossilized Raptor, the mammoths kind of fit the theme of the first gear set, corpseflies, Rivendare’s Deathcharger, Midnight, Headless Horseman mount, Spectral Steed (think this one is different for horde), Armored Bloodwing.
Depends on what aspect of death you’re themeing yourself on. Folks have set a lot of skull based folks which is good, but you can also look into some of the Maldrax and Revendreth mounts as they fit the heavy death theme we have. Stuff from the Maw/Torgast fit our theme as well. There’s also the blood ghost horse as well.
Could also go for any red dragon mount, cause reasons 
Im surprised they didnt make them fly for this expansion. Being spooky wraith horses, youd think theyd be capable of doing that.
My biggest issue with the skeletal gryphon is how the bone wings dont fold when on the ground.
My go to is the Headless Horseman’s mount. I was thrilled when I got it years back, and I feel it has aged well.
I mean obviously invincible is the most dk mount of all time imo. And any of the frostwyrm mounts. Icebound protodrake is good too (can you get that still?)
Rivendare’s deathcharger is more of an undead mount, but anything like that makes sense for dks.
I also always thought the mechano-hog was pretty sick for dks. We are edgy as hell after all
I really wish there were more deathcharger mounts. Actually on that note, why don’t the racial/class mounts have more cool customizations? Like dragonriding I feel like our racial/class mounts should have a bunch of custom options. But obviously that would take time and money, which is something that Blizzard has but doesn’t want to spend. Gotta get that CEO another yacht after all
I believe you get it upon WotLK classic release. If you missed out on it, I think you have to wait until trading post releases it.
You cant, had to make a DK during the early stages of Wrath Classic. I dont even think you needed a 55 character to make a DK at that point which bots flooded to the class, and then they had to change it later on because the bots were out of control.
To add,
For ground only I really like the spider mounts.