Timothy hay and other grass hays are the primary source of fiber (and also the main food group) for rabbits, guinea pigs, et cetera. Since I have a house rabbit of my own, I've put boxes of hay in my rat's play areas just in case they'd like to borrow through it/play in it. Though, hay can be very dusty (especially those small bags from the store) which can cause issues for rats that have respiratory issues.