You won't be able to move as quickly if you choose to lead the tortoise rather than ride. You'll fall behind the others, but it's not technically a race so that's fine.

Gan and Onizos, as you pull the reigns your tortoises begin lumbering forward, but as soon as you try to change direction to follow Adan they get agitated and start bucking. Please make a Dex Save to stay mounted.

Yabo, your tortoise plants himself stubbornly as you try to lead it. Make a strength check to try to get it moving.

Dex save 13

Gan, angrily holds on as tightly as he can to the reins of the horizonback.

"What's your problem, turtle?" Gan says. "Calm down or Is'll make soup out of you."

Dex Save: 23  

STR: 21

(Sorry had unexpected things come up on top of the holiday so things have been a bit hectic for me.)

Igreis moves to watch the others as they guide or mount the large turtle, hanging back to watch for the time being.

Igreis (and Baetrix) you're welcome to jump on one of the other tortoises and catch up at any point.

Yabo, you brace yourself against the side of the beast and give it a strong shove. It barely moves, stumbling a half step, but this is enough to break it's inertia and get it to start following you.

Gan, you nimbly shift atop your tortoise as it bucks, until eventually it settles down and lumbers through the gate.

Onizos, your tortoise bucks particularly hard and catches you off guard. You slip from the saddle and tumble to the ground taking 1 bludgeoning damage. You're able to climb back on, but it seems it's getting increasingly agitated at you.

Adan leads you through the Jumble and past gawking festival goers "Careful on this part, go through single file!" he shouts as you approach a narrow chicane between two buildings. 

Please all make another Animal Handling check to guide your tortoise through the obstacle.

Yabo tries to make sure his stubborn cousin behaves.

Animal Handling: 13

Gan waves at the gawking on lookers.

"You'd think they'd never seen no kobolds riding giant turtles before," Gan says.

Animal Handling: 15  

Animal handling 12

Onizos attempts a cavalier attitude as he rides the creature. “Yes, after living on one of your much larger cousins down in the swamp, riding a little thing like you should—oof!”

He picks himself up off the ground, wincing and rubbing his backside. He gets back on, muttering under his breath, cavalier attitude gone.

Gan and Yabo guide their tortoises carefully through the street.

Onizos, you misjudge the size of the space and the turtle crashes through a beam and the buildings awning collapses to the ground. The now panicking tortoise rears.

Make a DC15 Dex Save to stay mounted. if you fall you'll need to make another DC15 Acrobatics check to remount the angry tortoise this time.

This is getting kinda frustrating, such uniformly low rolls!

Dex save 7, Acrobatics 11, and yeah, he has +3 dex and +5 acrobatics

You try to scramble back onto the tortoise but slip and it's swings it's long neck at you and goes for a bite.

To hit: 10 Damage: 6

Onizos’s eyes narrow in anger and he mutters nonsensical words at the beast that carry an unmistakably malevolent tone.

He casts dissonant whispers on the foul creature, dealing 16 psychic damage to it and causing it to run away if it fails a DC13 wisdom save, or 1/2 damage on save.

Adan swings back around and helps calm it down allowing you to remount if you choose, "Gotta be careful with Rufus here, he was born ornery"

Eventually, the city gives way to a large pasture. On seeing the open field and fresh grass, your mounts happily gather in the large temporary pen made for them. Adan comes around to each of you, helping you dismount and thanking you for your help. "You are all now honorary horizonback herders. Be sure to come back and visit your new friend when he's all grown up and has a house built on his back"

He hands each herder a medal. Each is a unique piece of horizon back shell:

Medal of the Horizonback
When you would be hit by an attack, you can use your reaction to increase your AC by 5 until the start of your next turn, including against the triggering attack. You must be wearing the medal and able to see the creature that made the triggering attack to use this property. Once this property has been used, it can’t be used again, and the medal becomes nonmagical.

Wisdom Save: 2

Rufus runs wildly away from you Onizos brushing against several more buildings and causing not insignificant damage. Adan is still able to corral him a bit further down the road but on seeing the frazzled beast suggests he lead him the rest of the way to the pasture.

Adan would still be willing to give you a medal, but would require persuasion 

Gan hops off the horizonback, accepts the medal and hangs it around his neck, puffing his chest out a little as he does.

”Don’t think it likes you, Oni.” Gan says with a laugh. “What’s left? The swimming?”

Onizos doesn’t feel he’s earned the medal.

“Hmmph,” he says as he turns his back on the whole affair. “I never liked the smelly beasts anyway. They remind me of the swamp…”

Yabo looms up behind Onizos, clawed fists on his hips.

"If 'y cousins are s'elly 'easts that re'ind you of the swa'', I'll 'e sure not to sit u'wind fro' the high and 'ighty 'Og 'Ane, would't want hi' to 'e re'inded of his terri'le ti'e there.", he rumbles before turning and walking towards the river.

" 'Ay'e if I take a di' in the water, 'y s'ell won't offend so 'uch."

Those closer to him might hear Yabo grumbling about 'wet goat stink'.

Gan laughs at 'wet goat stink' and trots after Yabo.

"You hurt the big guy's feelings, Oni," Gan says. "Who'd have thought he'd be sensitive under that hard shell."

As he moves along quickly to keep up, Gan smells his own clothing, then shrugs.