He then comes Rainbow Dash behaving recklessly and tells her off, but she doesn't take her seriously.

Is it the one where he's twilights marshall?

this maybe ? https://www.fimfiction.net/story/482797/the-new-guard

7452708
I think there looking for the one where dash is in trouble because she tried to stop winter. He finds her with a buried tank.

7452708
That's the one.

7452704
He becomes Twilight's marshal?

7452723
Here's the marshal one. https://www.fimfiction.net/story/307030/do-humans-hibernate

Rainbow tries to stop winter and ends up breaking the machine. Marshal has been put in charge of her community service and makes sure that she does her job.

7452741
Wow...

7452748
It's a really good story in my opinion. If you're not into pony x human, then you might not like it as much, but overall it was a really sweet and well paced story that built up both of their characters rather nicely. I'd definitely recommend giving it a shot.

This human raised in Equestria becomes a royal guard.
https://www.fimfiction.net/story/443542/brighter-horizons

7452884
This one’s super underrated guys. You should really check it out.