DEFINITELY acorns are very bad. Recently lost an elderly girl to acorn toxicity and working hard to keep the other ones healthy since. I'm getting the feeling there aren't any safe trees for horses!Red Maples and Sugar Maples are definitely toxic! Two of our minis survived thanks to intensive ICU care at UC Davis Vet Hospital. Sadly, three weeks earlier, a sweet percheron draft passed due to toxicity poisoning although the cause and source of the poisoning was unknown at the time. Discovered that it was due to sugar maples. We were informed that many varieties of maples are hybrids of red maples. Also studies have indicated that pistachios and walnuts are highly toxic to equines. Although less toxic than others, oak leaves and all acorns can also be toxic and deadly to miniature equine.