Here's an idea to enhance Horizons massively.

Bipeds can purchase aerial mounts… They would limited to flight
speed of 75 (no items to increase flight speed) but would treat
flying much like dragon flight. They would be worn in the cargo
disk spot and when worn, the biped would appear mounted on his mount.

Mount Types:
Giant Bat
Has a bite attack instead of biped's melee attack (Lv. 50)
Giant Eagle
Has a claw attack instead of biped's melee attack. (Lv. 60)
Griffon
Has claw attack instead of biped's melee attack, better than
Eagle (Lv 70)
Serpent Wyrm
Has claw attack that is pierce based, better than Griffon (Lv. 80)
Phoenix
Has fire based claw attack, better than Serpent Wyrm (Lv. 90)

The mounts are quest driven and the quests are on par with the RoP
quest… must go through all 5 quests to get the Phoenix

Casters can cast and bows can fire from mounts but at 2x recycle and
wait
Dragons can use any of their abilities in flight, but also have 2x
recycle and wait times.
Dragons, if they so choose can wear a saddle in place of a cargo
disk which allows a biped to ride on them, but only the dragon can
do melee in combat. The rider has no control over where the dragon
goes though.

Land-based monsters would still run away much like they do when
someone sits in a tree and attacks.

Aerial Monsters:
For aerial combat there needs be aerial monsters. These monsters
stay above a certain height such that they don't attack ground-based
players, but do attack anything flying near enough to pull aggro.

Aerial Monster Ideas:
Blighted Giant Bats
Undead Eagles
Undead Griffons
Undead Wyrms
Ghostly Dragons
Black Phoenix
Shadow Dragon
Whirling Dervishes: Wind(crush) Sand(pierce) Blade(slash) etc.
Greater Kwellens: Same types, with bigger wings and having flight.
Many more could be created…