。
It's a living creature, so of course it's going to poop.
Literal
Living-thing is-[because-んだから] poop do [it's-certain-that-に決まってる][explanatory-んだろ].
Very casual, rough speech. ~に決まっている ('it's decided that, it's certainly the case that') is the fixed expression for 'obviously, of course.' The contracted んだろ = んだろう. ウンコ ('poop') is a childish/casual word written in katakana for emphatic crudeness. ~んだから is the explanatory reason-giver, stronger than plain から. The whole utterance is a rough retort — rough masculine register.