Once per turn: You can target 1 face-up monster you control; it gains 1000 ATK 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 Tree Otter as 1 of the materials, you can Special Summon Tree Otter to the Field using Powerful Rebirth, and since you control Koalo-Koala, you can use Tree Otter’s effect to increase Koalo-Koala’s ATK by 1000.
Your monsters gain ATK
Interesting for Beast-Type decks. Note that you can also use the effect on itself, you just need to control another Beast.