In that case, I think a fathier might be a good choice. They’re quite expensive and may be hard to acquire, but they are definitely fast and agile and there is an instance of the Rebellion using them for hit-and-run missions. A falumpaset might also work, as they’re intelligent and used to navigating jungle and swamp, though they won’t be as fast – they may be better used for carrying equipment/ammo. Many ungulate species may also work, depending on where in the galaxy you are, especially the kybuck, a Kashyyykian animal resembling a tauntaun. You might like to look through the ungulates category on Wookieepedia, and see if anything there fits your location and circumstances.
I hope that helps!
~ Jacen