You could try going about it a two ways I just thought up of in my head
1: Spawn multiple cows (depending on depth), each giving 1 leather. Probably the easiest to code for I'd wager.
2: Spawn a different colored cow or type of cow (like a bull or something bovine related) depending on depth entered. Each rarity of cow giving a different number of leather. Might have to add new assets or code in some coloring, so this is a probably the harder option.
Either way, really fun game!