Hi,
I am new here, so I am not an expert, but a friend of mine told me his experiences.
So, as far as I know, it is not good if sheeps eat horsetail.
It grows in wet areas and if sheeps eat it they can get dizzy.
He told me that the consume of to much of it causes strumbling, jumpiness and even paralysis.
But the sheeps realy need to eat a lot to experience this suffer.
Just a bit of one plant is not a big deal, I guess.
If you are just worried that your sheeps try one plant were you hold them, this should be ok. But if there is mostly horsetail growing it is not a good place for them.
Hope this is helping