Stealing a horse is different from their usual heists - and Inej is the only one to have ever properly ridden one at all like this. Jesper has worked with plow animals, of course - but Inej has ridden spirited beasts before, ones used to the energy and speed of performance.
Which means that, when the guards catch a glimpse of Inej attempting to lure the stallion with his gleaming black coat out of his stall, she simply climbs to the top of the half wall and jumps on his back. He rears - snorts angrily as his front hooves touch the ground - she laughs and adjusts her balance with him -
And he bursts out of the doors she left open, running down the hapless guards with a powerful scream. Inej has no saddle nor bridle on him - she doesn't need it, nor does she need him trying to bite her while she puts them on - and she simply declines to fall off as he leaps over a fence - breaks into a gallop - slows to weave among trees -
Inej is still laughing when they approach the meeting space. It's a good ways off - she insisted on that, even if it meant backup would be far - and he's run himself tired enough that he doesn't even make a threatening gesture at Kaz and Jesper, just huffs and stops in front of the watering trough she asked Jesper to find and fill.
She jumps nimbly off his back, gives him a friendly pat, and asks Kaz - "Did you get the combs and treats and all?"
(A horse needs to be groomed after a good run like that, and anyways, she wants to check and clean his hooves while he's too busy drinking to attack her about it.)