Various items can dismount enemy players, but only from flying mounts. They work on druid travel form and soaring Drakthyr, but only if they are mid-air.
Net-o-Matic 5000 - Usable in all zones, including the Dragon Isles. Slow cast, but long range and short cooldown. Fires a dismounting projectile.
The cast can be broken by interrupts, or breaking line of sight/leaving the Net's range for the moment it would fire.
The user is put into combat when the Net-o-Matic fires, so it can be difficult to reach distant dismounted enemies without other mobility tools.
Purchaseable at multiple vendors for 25 Marks of Honor, or can be obtained from a Battle for Azeroth or Shadowlands quest. (See WoWHead page linked above)
Qalashi Wingshredder - Only works in the Dragon Isles. Puts the user in combat immediately and requires you to aim your camera at the enemy while casting. Fast cast, short range, requires line of sight for the whole cast, and has a long cooldown. Dismount can be prevented by reacting with Whirling Surge.
Salt-Hardened Shell - Only works on targets below Level 49. Drops from a Mists of Pandaria enemy in the Dalaran Sewers.