It's not that nothing like this exists in nature, it's that evolutionarily speaking nothing like this existed in the areas human first evolved. Or if they did, we either evolved to resist them, thus no longer see them as toxic, or we're still in an arms race with the life that produces them (which means they would be much less toxic than fentanyl, but still toxic).

Plants (and fungi, and animals) evolved to create noxious chemicals that keep animals from eating them, keep animals from breeding, or outright kill the animals. The Golden Poison frog's toxin is about 40 times more lethal than fentanyl.

The golden poison frog looks pretty scary to me. The yellow color. The big eyes. I would not try to eat it. But a white powder or pill that is equally lethal doesn’t ring any evolutionary alarm bells.

Could you please link to or expand about plants producing toxins that specifically keep animals from breeding? It's news to me, and a web search has failed me. Is that long-term pest control, or something else?

The term you want to search for is "phytoestrogens".

From wikipedia: "It has been hypothesized that plants use a phytoestrogen as part of their natural defence against the overpopulation of herbivore animals by controlling female fertility.[3][4]"

Though wikipedia also says: "These compounds in plants are an important part of their defense system, mainly against fungi.[19]"

So it could be that plants are principally targeting fungi for the most part, and just serendipitously targeting animals as well.

I'd say the opposite, nature creates all kinds of powerful attractants and poisons. There are plenty of things that will kill you at the micrograms dose. There are plenty of natural things that are horribly physically addictive.