These hormones occur naturally in the body. We found an analog in nature that behaves a bit differently in that it is longer lasting. Now we have synthetic versions based on that understanding.

And the gila monsters have their own circulating serum levels of exenedin-4. It's produced in their salivary glands - it's a component in their venom, yes, but they also ingest it themselves, particularly when eating.

But these are fundamentally analogs to our own hormones that have been tweaked to increase desirable properties. It's not like we isolated some random compound that our body has never had to deal with before. (Not that I think that that would necessarily be an indicator we shouldn't use it, either. I don't see why any of this is a strong argument to you, even if your understanding was correct.)