Shrubbery

For those interested, enough caffeine to kill a horse is greater than that for a human. This is assumed to be true, but here are some sources.

A fatal dose for an average adult is about 10g.
https://en.wikipedia.org/wiki/Caffeine#Overdose

This source investigated caffeine intoxication of horses. There were days where the horses consumed more than 10g, they had symptoms that were alleviated once their source of caffeine was removed.
https://pmc.ncbi.nlm.nih.gov/articles/PMC3317824/