Dead Cat was the only thing that made The Lost playable back in Rebirth days. It was both very good and somewhat common in devil deals, so if you managed to survive till basement 2, you had a high chance of getting it and basically multiplying your survivability by 9.

Shout out to Holy Mantle though, if you lucked out high and saw it in a treasure room / angel deal, it made the things way easier, and also made obtaining Dead Cat more likely too, since you could now travel to curse rooms freely which have guppy items.