Assuming a mythical creature is functionally a natural biological entity, then its physiology, metabolism & general biochemistry will be unlike either a horse or a human. However, it may be a mammal. So what poisons horses or humans might poison a centaur. of course, the other hand what will poison either humans or horses may not poison a centaur.
The preceding remarks were intended to clearly establish we are in realms of speculative toxicology and conjectural biology. This means the correct answer can be: perhaps.
However, it really depends on the nature of the poison proper. Many poisons will kill mammals -- this includes both humans and horses. So definitely not. Now if there are poisons which kill, say, humans and not horses, and, on the other hand, poisons that kill horse but not humans. However, either of these types of poisons won't lead to any easy answer.
Why not? Because we're considering a creature that is strictly speaking neither horse nor human. A centaur may be able to ingest and safely digest a toxic material or substance, say some kind of poisonous plant, that would kill a man, but a horse, and definitely not a centaur.
In conclusion, while many poisons, for example, arsenic, cyanide and strychnine, will kill humans, horse, and presumably centaurs, there could exist a class of poisons lethal to humans, but harmless to horses and centaurs. But you would need to understand the mechanism of its toxicity and how it affects the different types of creature.