ethoxyquin
Ethoxyquin is a synthetic antioxidant preservative that has also had agricultural uses. In 1997 the US FDA asked manufacturers to voluntarily lower the maximum level in dog food from 150 ppm to 75 ppm, and in 2017 the EU suspended its authorization as a feed additive due to insufficient safety data. It remains legal in the US; if you prefer to avoid it, look for foods preserved with mixed tocopherols (vitamin E).
Other common names: ethoxyquine, EMQ, santoquin, 乙氧基喹啉, エトキシキン
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Sources: ASPCA Animal Poison Control Center, FDA Center for Veterinary Medicine, veterinary literature.
For reference only, not medical advice. If poisoning is suspected, seek veterinary care immediately.