Rescue Cat

Seems like a harmless card.
Don't make me laugh.
This is one of the most dangerous card in the game.
Giving you the ability to summon not 1 but 2 monsters from your deck which is way too hyper.
Having the ability to access most monsters in your deck, this card is way too good to be even limited.

Provide easy rank 3 or lower xyz summon or lv 6 or lower synchro summon is bad enough.
Considering that it summon monsters from the deck which thin the deck by 2 selectable cards improves the possibility of drawing the cards that you need next turn. Not to mention it can be special summon from the deck by cards like Giant Rat and Summoner Monk. Also, can be revive to be reuse by cards like Monster Reborn, Call of the Haunted and Limit Reverse.

This cute little can make deck likes X-Saber or Unicorn way to strong. Having this card alone, gives you the ability to summon X-Saber Airbellum (Lv3[Tuner]) and XX-Saber Dark Soul (Lv3) which can be use to synchro into XX-Saber Hyunlei (Lv6) or other Lv6 Synchro. Which means, using this card alone, allows you to call forth a monster with 2300 ATK with the ability to destroy up to 3 spell or trap cards on the field due to XX-Saber Hyunlei's effect and during the end phase gives you the ability to add an X-Saber monster from your deck to your hand due to XX-Saber Darksoul's effect. Other cases, is with Unicorn deck, which you can choose to summon Unihorn Familiar (Lv2[Tuner])/Elephun (Lv2[Tuner]) and Mine Mole (Lv3) to synchro to Thunder Unicorn (Lv5) which can reduce the ATK of opponent's monster by 500 for each monster you control. Usually,it's not quite easy to summon Thunder Unicorn, but Rescue Cat has made it way too easy to summon it. In addition, due to Mine Mole's effect, you get to draw 1 card. It's almost like you didn't use any card from your hand at all to summon Thunder Unicorn.