I'm about to launch into making a few folding runs for small animals
(rabbits/guinea pigs) which will necessarily spend a lot of time sitting
on the grass.

The design I have in mind only needs a 1x1 frame, so that just about
brings nice woods like oak and ash into a reasonable price range.

What's the best wood to use for this application? (possibly with some
pet-safe preservative, but I'd like the wood to have some innate
resistance to rotting as well).

And as well as the *best* wood to use, what's the *sensible* wood to use?
I know there's probably some exotic species that wouldn't rot for 100
years, but at a ridiculous price...