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Plants may be toxic to animals for a number of reasons. If a plant is unpalatable or causing sickness in the insects and animals that would browse on it, then it can be a great advantage for that species. On the other hand the toxicity of the plant to mammals, or even just to horses, may be a by-product of producing substances which attract insects or animals to its seeds or flowers.
Bracken Fern
Buttercup
Castor Oil Plant
Deadly Nightshade
Foxglove
Hemlock
Linseed
Potato
Ragwort
Rhododendron
St John's Wort
Yew