Teasel (D. sylvestris), if this is the right one, appears to be used by some in the form of a tincture made from its roots.
So perhaps if the roots are not poisonous the rest of the plant may not be.
There's mention of someone feeding to their tortoise but heaven knows why when there are better plants around for this. Too prickly I would have thought.
Other than that I can't find any information about its toxicity to animals.
An interesting question.
