The perk itself is a reference to 50[MINISTER]56ms' fans calling him a rat, which originated from him titling streams "Rat Hunting" while playing the game Escape From Tarkov.
It is possible to capture a Rat soul by using an item that has the Raise Souls ability. This is the only soul with 0❁ Damage and also the only one that costs 0✎ Mana to summon. The soul will appear as a Zombie after it is summoned by a player.
There was a common bug that made a rat's damage box sink into ground, making it almost invulnerable. Some workarounds were using a Shadow Fury, Explosive Bow, or Phantom Rod.
Killing a Rat actually counts towards the 10 Zombie kills for the Time to Strike Questline.