Looks like cow parsley, which as a member of the carrot family most equines enjoy grazing on, but it's not. This is Hemlock which is deadly poisonous, it's what the Greeks used to execute Socrates, so definitely not something you want growing in your pasture or dried in your hay. Even getting the oils of this plant on you through handling can result in poisoning.
Generally speaking horses won't eat it as it has an unpleasant smell but if they do it's likely to end in death so it's important that you learn to recognize the plant.
Hemlock leaves are darker than cow parsley and more feathery in shape.
The smooth stems have purple blotches often more prevalent at the bottom of the plant.
Death usually occurs due to respiratory paralysis followed by cardiac arrest.
Consumption of Hemlock is a medical emergency and URGENT veterinary assistance is required.