Mounting should be fairly self-explanatory, but what are tandem mechanics?
Tandem mechanics are a kind of soft-fusion if you will, where two creatures act together simultaneously. We'll elucidate further in the relevant subheadings for these mechanics.
Mounting a creature is, by default, a Complex Action. When mounted on a creature, you utilise that creature's Movement Allowances to move rather than your own. However, your own movement allowances are still consumed while mounted, so you are not able to fully utilise your mount's movement, dismount and then consume your own afterward. This is still all happening during your activation after all.
For the purpose of rules which cite 'Your movement allowances' or similar phrasing, you consider the mount's movement allowances to be your movement allowances while riding it.
Normally, a mount may use the Rush action on it's own activation, if it has one (Mundane bicycles are an example of a mount which lacks it's own activation). However, if you have the Urge Mount perk, you can instead use your Ride skill to assist the mount in covering more ground, rather than using the Mount's relevant mobility skill.
In order to comfortably mount a creature, it should be at least one size category larger than you, but the GM may override this stipulation in situations where they feel that it is appropriate. In terms of creatures a mount can comfortably bear, it can carry up to it's Weight Class plus it's Lift skill divided by 10 (rounding up) of Weight Class from Riders. An overburdened mount gains the Slowed condition until it is no longer overburdened.
F.e. A human woman may have a weight class of 7, and can comfortably ride a Rapidash, alongside her weight class 9 companion, but said human woman could not comfortably carry a Weight Class 16 Cosmoem no matter how strong she was.
Vehicle mechanics are still very much in their infancy, but in effect you can consider a vehicle to be a Pokemon, and they should largely be handled as such (with most vehicles being Varoom/Revavroom/Rotom forms as the intent). However, perks that interact with the Riding Skill do not apply to motorised vehicles even if they do apply to a mundane bicycle. We will aim to introduce piloting perks that key off of the appropriate pilot skills for various vehicles as these become more developed.
Grappling is covered in greater depth but mentioned here to direct your attention to where it makes an appearance in Attacks and Actions. A grappled mon is essentially involuntarily mounted on the attacker, and carried along as a passenger who may not simply dismount without breaking free.
The meme made real, folks. In PTR2 you can perform the vaunted feat of "casting Yeet" and flinging your allies, or even grappled enemies, at your desired target. For those whom feel the need to work out the yield of such a maneuver, the mathematics can be found on the Fling move itself to see just how far and how hard you can accurately throw your friend/victim, but for the purposes of this rules explanation, we will not be including the calculations here.
To utilise this, target the desired enemy you wish to throw the creature at, and then initiate the Fling attack. Select the Actor Toss variant option, and then Foundry will prompt you to hover over the actor you wish to throw, and press the T key. This should lock in the projectile, and you can proceed from there with your desired mayhem, resolving the fling attack like normal.
There are two forms of Wielding a creature, one of which is an extension of the Fastball Special so we will cover it first. Should your deserving victim be within your normal melee reach, you may opt to utilise the Actor Toss variant of fling to use your chosen bludgeon without releasing your hold on the creature.
The other, rather more respectable format for wielding a creature is through the use of the Living Weapon trait. These creatures have been given specific designated weapon profiles that you can find in the Equipment compendium folder, and are considered to be riding you as a mount for the purpose of their own activations. Naturally, these creatures cannot use their weapon profiles themselves and must rely on the actor wielding them for that, but they can use any moves that would seem reasonably viable, as they are not constrained by the limitations of the Grappled condition. GMs ultimately have final say on if a move is reasonable for a creature to use whilst being wielded.
Wearing a creature typically comes about as the result of a very specific trait or ability, which will give specific instructions as to where it may differ from the standard ruling, but typically this is used as a vehicle for "fusion" type mechanics, where the creature is functionally converted into a piece of equipment and occupied one of the actor's equipment slot, as designated by the trait or ability that allows the creature to be worn as equipment.
By default, the creature becomes mounted on the host actor (and in the case of fusions becomes weightless) but normally the creature will be removed from the initiative and may not be directly targeted by other actors on the field.