So, a character has a Mature Adult Black Dragon fast zombie cast through Animate Dead (ruling that it counts as x2 the HD).
1) If I slap a saddle on it, would I still use the ride skill to ride it?
2) Being large, it gets +2 HD. I assume these new HD are counted towards how many HD I can have under my control at any time, right?
3) Does it lose it's normal natural armor, and only get +3 for being large? Or is the +3 in addition to its normal natural armor?
4) I don't see it anywhere, but even if it is a fast zombie, it's flying speed still drops to clumsy. Right?
5) Can I equip this zombie mount with magic items (maybe treating it's available item slots as you would an animal companion)?
5b) Could I equip it with a Amulet of Mighty Fists with the Training weapon enchantment? I don't know if the Run feat works with flying, but if so, dope. Otherwise, Hover would work, right?
5c) What about a headband of INT +2? I would still control the undead. Would it have animal intelligence and 1 rank for Fly per HD?
6) A portable hole can open 6 ft. in diameter, and is 10 ft. deep. Surely I can store my mount in there, right? It doesn't need to breathe after all. And how quickly could I get my mount out? I'd assume move action to get the hole, standard action to open it? Then free action to tell it to fly out. I guess next turn, I'd need another move action to mount it before I could fly away, though.
1. I would normally require it be an exotic saddle, and I would apply the –5 penalty to Ride checks for having an ill-suited mount. They have no intelligence, aren't trained for it, and generally require you to give spoken commands. So you'd use Ride for most purposes; staying in the saddle, etc. but other things, like guiding it with your knees or spurring it faster are not going to work.
While it will listen to your commands to move or go somewhere, that's not the same as how mounts are usually guided. You can certainly give it a long term trigger or a condition (as long as it isn't complicated), like, "If I do [this], then you charge forward" or "If I squeeze my knees on your side, you turn (or move) in that direction." As unintelligent creatures, I wouldn't allow them to retain a whole bunch of such commands (definitely not as much as a trained horse or dog), otherwise you fall into the training aspect. One or two specific actions would be fine, but that's IMO. If you give them too much, they will just stand there trying to sort out too many branches of variables and outcomes. So maybe a couple commands that you trigger an action based off a non-spoken action... but that's not really Ride (in so far as controlling the mount).
2. Yes. A Mature Adult Black Dragon is typically 16 HD, with a +2 increase for being a Large Zombie. So 18 HD or 36 effective HD if you double the HD cost for the Fast Zombie template. You'd need to be a 9th-level caster (4 times your CL) or cast it within a desecrate spell. So that all seems doable and within reason.
3. It appears to lose its Natural Armor and use the Zombie's listed bonus. Most of the example creatures I have looked at seem to use the zombie's listed Natural Armor bonus without adding it to their own.
4. The fast template appears to modify the zombie template. Since there's no mention of Fast Zombie affecting fly speed (like there is for land speed), then it's the same as a normal zombie.
5a. Zombies can equip and use magical items as appropriate for their form and body (how well they use them, like a sword is a different story). An amulet of mighty fists would work if a normal Mature Adult Black Dragon had the body type and ability to wear one. The zombie's unarmed strikes and natural attacks would benefit as normal.
5b.1. The training enhancement appears to be a legal addition to an amulet of mighty fists.
5b.2. A flying creature can run (as opposed to a fly spell that lets the target charge but not run). The Run feat does not specify only Base Speed or land speed. A flying creature can run and they can benefit from the Run feat. However:
5b.3. Neither Run nor Hover are combat feats, they are listed as General and Monster (respectively) and are not valid as options for the training enhancement, which must be a Combat feat. However, hovering is an action that can be done by any flying creature (but it will need to make a Fly check, which is not going to be great, being a zombie and all).
6a. If a Mature Adult Black Dragon can fit in your portable hole then the zombie version can. I would say that the dimensions given seem acceptable for a Large sized creature to enter (whereas a Gelatinous Cube, another Large creature, is listed as having 10 foot dimensions on each side, and I would not allow it to fit into the 6-foot wide opening).
6b. No indication of actions or time given in the item, which means it falls to common sense and GM discretion. Personally, I would say it's a move-action to retrieve like any other item. Then I would likely say it's at least a full round action to lay it out and open it (and provokes AoO). This is because the description seems to indicate it most closely would resemble laying out a picnic blanket or tarp. You aren't just flinging it out like a Poke' ball or one of those easy pop-up personal camping tents. You have to unfold like a handkerchief (then like a sheet as it gets bigger) and spread it out. I would say at least a full-round action (I mean, that's 6 seconds), about the time it might take to drape a king-sized bedsheet over your bed and let it settle down and lay flat.
Re 5c: A zombie has no Int score, and fast zombie does nothing to modify that. Therefore, it cannot meaningfully benefit from a Headband of Vast Intelligence: It does not have an Int of 0, it has an Int of -, and - plus any number is still -.
That said, as written the ability to gain skill ranks does not seem to be contingent on actually benefiting from the Int bonus. I think it is supposed to be, and would not allow it to work, but that might technically be a houserule.
Re 6b: When considering the action economy of retrieving your mount, do not forget to factor in wrapping the portable hole up and stowing it again. Portable holes are expensive, so you are not going to leave it lying on the ground....