Said before looking at a box of animals to see if they are still alive:
「今ならまだ一匹しか死んでいないものが、次の日にはまた一匹死んでしまうかもよ」
If I (look) now there will still be only one dead, but the next day another might end up dead.
I don't understand what もの does here.
Actually I think I must have messed up the first clause altogether, since we don't yet know if any of them are dead. Literally I translate this as "As yet, with the exception of one, they are not dead".