I'm working with a scene that wants to play out as follows: a disguised changeling is exposed to what should have been a non-harmful pesticide (it's harmless to horses), but ends up having enough medical trouble that emergency hospitalization is required.

The problem I'm having is that I can't come up with such a pesticide. Everything I'm finding online is either horse-toxic as well, or kills so quickly there wouldn't be enough time to get the changeling medical help, and I don't want her to die.

Anyone got any suggestions for a good pesticide to start with?

5418450 Well for one, MLP ponies and own horses are the the same animal so that may be one of your problems. Simple solution make something up. Go to a random name generator, get a name and them decide what it does. Done.

5418450 Cypermethrin is primarily a fly-spray and horse-safe. So is permethrin, though it also covers mosquito's.

Actually, lemme just refer you here.

https://entomology.ca.uky.edu/ef513

The changelings could just be allergic to something that ponies aren't or digest differently. There are a variety of spices that we humans enjoy that are downright poisonous to other animals, nevermind all the weird allergies that crop up in ourselves. Cinnamon, nutmeg, garlic, cocoa, pepper and a lot of others can all be toxic to a variety of creatures, so it's just a matter of taking a pick of one and playing it up. (Of special note is cinnamon, which is supposedly an ant repellent.)

There's also that one scene in the Canterlot Wedding that could lend credence to it - the one where Chrysalis refuses Applejack's treats because reasons. You could play it up as her not being willing to take the risk.

Most likely option is to make something up. Just name it something like dicarbontripolymhexaoxide. Big writers do the same thing all the time so it's not too strange. Just look at the last episode of mlp in the beginning.

BT, assuming your lings have the appropriate biology to be affected by it.
They could probably live a relatively long time with their stomach liquifying itself.

Could be a magic-based pesticide...

Also, I'm pretty sure even a horse wouldn't be killed immediately after, say, consuming pesticide-treated leaves. It'd probably be a slow painful death.

5418450
Consider horses to be what they are: placental mammals, just like us. Now, consider that most insecticides are made to be pet-safe, and that people keep as pets from fellow mammals to birds, reptiles, amphibians, fishes and even some odd cases like me, insects and arachnids. Insecticide manufacturers try to then design their chemicals to be not only to humans, but at the very least all hot blooded organisms, and preferably all red blooded organisms. Of course they aren't entirely safe, but these chemicals target the difference between a white blooded and a red blooded organism, so foul-smelling fragrance to one is pure poison to the other. Think of it like the difference between being allergic or not to peanuts: either you can eat them by fistfuls or they are lethal to you, with no middle ground.

To then answer your question, go to your supermarket and pick a general purpose insecticide, then say that this product in particular is good to kill, but nowhere as fast-acting as, say, roach-specific insecticide.

For physical one that was actually used a LOT and had many such applications, DDT was long thought to be safe for higher animals like humans, horses and cattle. Its long term dangers did not come to light for over thirty years after it was introduced.

That could make it a good candidate for your changeling to get hit with. Now banned. Some found in a farm storage, Ponies involved read the OLD label and follow instructions.
Your changeling gets some and Oops!
Total accident, for real.