I'll go over everything then, why not. I've turned into Strat's personal homebrew reviewer so I can do that for you also, I figure I've helped enough with other projects of yours to turn my attention tothis.
We'll start with the Death Knight.
I'm AFB, I'm going to assume the maneuver and stance progressions are appropriate.
The capstone could probably use Cha to Hp since it's a melee class.
Other than that it's a solid class, nothing exciting but it fills its theme well.
As for the mount list...
You should have some text about how undead steeds can't be turned while being ridden by the Death Knight (like in the
Bone Knight PrC).
According to something I read (I'm AFB), DMG page 204 has paladin alternative mounts. Paladins can get a Unicorn at 6 so Fiendish Unicorn at 8 isn't out of the question (although yeah you want some text saying that the anti-evil abilities are now anti-good). You can also probably get away with having Nightmare as a lower level mount, just say it doesn't get the astral projection or etherealness abilities. Say around 8 since that's when the other flying mounts come into play (that's a gut reaction).
Yeth Hound is pretty much superior to Hell Hound in all ways.
The megaraptor feels kind of forced.
I don't see any issue with the skeleton options, same with the zombie ones (zombies are only good as mounts, can't even get good combat options there).