About 10-5 questions about mounts.
1. Ostriches are rideable in real life, why aren't there ostriches in DND?
2. If you awakened your mount and you're friends with it now, then you have an argument, and the cleric uses a beneficial spell on you both, does the mount get affected even if it might no longer be considered your ally?
3. What really happens if your Nightmare FoWP (Figurine of Wondrous Power) disobeys you, you mount it, and the 2 of you go to Hades? What happens there? Why on Hades and not on Avernus, since the nigtmare is a fiendish being?
4. So you're part of a long-lived species and raised a dragon from an egg. The dragon is intelligent, but regards you as someone friendly and lets you ride it. Does this override "intelligent creatures act independently" from the 5e Basic Rules?
5. Does your (awakened) mount have attunement slots? Otherwise, how can it benefit from stuff like a Periapt of Health?
6. So a bad guy permanently polymorphed you into an intelligent mouse/weasel/squirrel etc. Does your mount still accept you guiding it? Does it not recognize you?
7. What happens if you cast 9th-level geas on the BBEG's mount and command it "to serve you loyally", but the BBEG themselves escapes and later casts 9th-level dispel magic on the mount?
Please help me.
1. Just because we don't have stats doesn't mean we don't have ostriches. In 99% of the situations you only need to know the speed of a mount, which you can choose based on similar creatures. I do not recommend to have mounts participate in combat, much less be directly controlled by their players, so I wouldn't worry about lack of stats before you really need them.
2. I would not count mounts as allies, but
awakened mounts are meant to be more like NPCs and therefore I would count them as allies. However, I also think that the concept of "ally" is non-technical, and it's up to the caster to choose the spell's targets.
3. Nightmares come generically from the lower planes, Hades is one of them (if you want to go along with some canon). What really happens there depends on your DM.
4. Why would this? The dragon acts independently anyway. A mount moves together with the rider for sure, but they still act independently.
5. Yes, attunement generally applies to every creature.
6. I doubt that a mount can recognize someone was polymorphed, particularly because in those forms they can't even speak. In fact, it's not obvious that an intelligent creature would recognize it either. A generous DM might decide the mount can instinctively do anyway.
7. I am not sure... it seems people are confident that a 9th-level Dispel Magic doesn't work on a 9th-level Geas, but I think it would work. The RAW says that 9th-level Geas will last until ended by Remove Curse, Greater Restoration or Wish, but it doesn't say it cannot be dispelled.