most likely they'll colic. they'll nip and paw at their belly, and roll on their back. if you see your horse colicking, call your vet immediately, and try to get your horse off the ground. you can help your vet by identifying what your horse ate that was poisonous.
It's dead.
He accidentally ingested some poison that was sprayed on grass to kill weeds near where he was stabled.
Eaten a what exactly?
The horse rolls on bits for a long time.
It dies.
fever
take it to the doctor!
they start acting up. they get sick.
Good question. Every horse owner/lover should know the symptoms of a poisoned horse. It's important that you know your horse. Learn his habits, his facial expressions, his body language. Some illnesses can have very subtle signs at first. With poison there may be a very small window of opportunity to get help for your horse. Watch for signs of listlessness, lying down or lying down and getting up repeatedly, sweating, chills or tremors, nipping at his sides or flanks like he has colic, drooling, drooping head and/or eyes, foaming at the mouth. He may have some, all or none of these if he has ingested something noxious. Since horses don't have the ability to regurgitate (vomit) it can be difficult for him to rid his body of poison. At the first sign of illness call your vet. This is not a 'wait and see' situation, and because it can look like colic, which is also very serious, don't second-guess yourself. Keep your horse as comfortable as you can until the vet gets there. Take all food and water from his stall or paddock until the vet says its ok to feed and water him. And lastly, try to remain calm. You will be more help to him if you use a quiet, calm voice.
It can look very dull and sick. It can also act strange and hang its head low. But if suddenly one day your horse does eat a poison plant, look for anything funny. But to also make sure that doesn't happen, clean out your horses paddock daily and pull out any weeds or poisonous plants!
You can tell if he is acting weird ,and is making this weird food or something in a pot or something, and you can tell if it smells weird. Not all poison has taste or smell. If you think some-one is trying to poison you, dont let them near anything you are going to consum.
The most popular signs are the horse is ether rolling or pawing the ground continuously. If they do this call a vet right away.