All three of my horses stuck there noses in stinging nettle (monkey see, monkey do, I swear). They had some irritation for a day or two, but some ointment on those cute noses cleared it right up. The worst was my mare though, she then went and rubbed her nose so it got even more irritated and scabbed over. My guess was that they where all grazing in the same area and ran into a patch. Ouch! I bumped into it once, thought I had found a bee hive.
Nettles especially love poultry dropping soil, the place where I keep my horses has quite a few chickens
Nettles are apparently used in making fertilizer or something along those lines. The ones here seem especially nasty. I got stung by a wasp and first I thought it was nettles!