"I'll rein her in if needed."

"Oh, I bet you will."

She tugs on Birgitta's reins demonstratively, before investigating where they're tied. Just in case she'd rather hold them herself, of course. 

It's very easy for Maellys to take them up.

Only appropriate. 

Entirely natural.

And respectful of the roles their god has seen fit to place each of them in...

Properly obedient.

Time to go?

Seems so.

Outside, and they can see about a proper horsegirl.

"Transform, lovely."

She obeys.

Dark mist swirls around her, cloaking her human form and parting to reveal a tall black horse, rearing up on its hind legs.

"Wondrous," she murmurs, awe-struck, and touches Shadowmere's neck.

She nuzzles in.

'Being a horse is much better when there's enough room for me to stretch out.'

"You're even more beautiful with that room, too."

She essays a little prance.

She sets out to examine Shadowmere fully, murmuring words of praise for her perfect form.

Mmm. That's good.

As expected, given Shadowmere is such a good horse. 

Prance!

So does Maellys want to go for a ride?

She'd love to. 

Shadowmere is at her service.

She swings herself into Shadowmere's saddle.

And off they go!

She laughs in delight as the world falls away under them, steering Shadowmere to a barrow carved into the mountains north of Whiterun.