From UOOutlands Wiki
When a player is currently riding a mount, that mount's effective Control Slots is reduced to 0, meaning a player riding a mount may still have 5 control slots worth of tamed creatures at their command. If a player dismounts, and then exceeds their control slot limit, they will not be able to command any of their tamed creatures until they remount their mount or release one of their followers and are at 5 or lower control slots.
Creatures have two types of abilities listed: Regular and Passive. By default, creatures can use only one Regular ability every 15 seconds (the Haste trait can improve this cooldown, however). Creatures can use Passive abilities as often as they are triggered or relevant. There is no cooldown for Passive Abilities.
Player vs Player
All damage (not just from abilities) from tamed creatures to players is reduced by 50%. This includes damage from melee attacks, spells, poison, or abilities. When tamed creatures deal any form of damage to a player (melee attacks, spells, abilities, poison, etc) rather than having a 100% chance to interrupt the player's spells, tamed creatures instead have a (Damage Inflicted * 1%) chance to interrupt the player.
Any time a tamed creature attempts to apply poison to a player, there is a 50% chance the player will be immune to it/ignore its effect. This includes normal poison attempts from Melee attacks as well as Poison applied from ability effects.
When tamable creatures spawn, each of their Tamed Stats and Skills is randomized between -10% to +10 of its normal base amount. This means that players can potentially search for the "best" version of a creature out there, one that has a large number of "positive" bonuses to stats and skills.