I have tried [item2=Candy Hearts Box], [item2=Chocolate Rose Bouquet], [item2=Deluxe Survival Kit], [item2=Edible Tree Tree Cookie Decoration] and [item2=Rations Pack] none of them decreased my pets hunger in fact some increased more than 1+ points xP

Kosmo's hunger is now 4/10.

Fed him a Chocolate Rose Bouquet (notes from SubetaLodge: Decreases pet's hunger by 3)

Fed him a Rations Pack (notes from SubetaLodge: Decreases pet's hunger by 10)

I'm not too sure if "decreases hunger" is meant to mean it makes the number lower (and actually increases how hungry they are), or if it decreases how hungry they are by increasing the number (I'm probably overthinking this!).... it's a bit of an ambiguous phrasing for me!

This makes a lot of sense! Even more support in that case for 's suggestion about ways to increase pet hunger so they can eat more!

yeah "decreasing hunger" means the pets become less hungry, not that they can eat more. There's a whole topic of foods that count for more than one point at a time also: Foods that Decrease Hunger More Than 1 Point

As for making your pet hungry, you can use the appetite renewal pool at the bathhouse once a day. Using in battle with a different pet can also randomly drop a pet's hunger to 0. but the effect is random and it's a tier 6 weapon so that might not be the most helpful option