Once per turn: You can target 1 monster your opponent controls; change its ATK to 0 until the end of this turn. You must control a Beast-Type monster (other than this card) to activate and to resolve this effect.
After you Fusion Summon Koalo-Koala using Sea Koala as 1 of the materials, you can Special Summon Sea Koala to the Field using Powerful Rebirth, and since you control Koalo-Koala, you can use Sea Koala’s effect to make 1 of your opponent’s monster’s ATK to 0.
Good thinking, but if you have both of them on the field at the same time, you can reduce their ATK to 0 and raise one of your monsters' by 1000, allowing you to deal 2200 points of damage with just those two cards.
If you actually want to use an Ojama deck, Sea Koala gets good synergy with them since they’re all Beasts as well and he at least has...slightly better ATK when Ojama Country is active
You can send this card to the Graveyard; Special Summon 2 Level 3 or lower Beast-Type monsters from your Deck, but they have their effects negated, also they are destroyed during the End Phase. You can only use this effect of "Rescue Cat" once per turn.