youngrasher - rhodies are a large woody shrub to small multistemmed tree, have very thick dark leaves, shade everything else out, very showy flowers, mostly pink in Spring/early summer. Spreads all over. It's certainly poisonous though I've no idea if pigs would eat it or just root it out. Yes, post some piccies.
For other poisonous trees moprabbit, I'm not sure. The castor oil plant (ricinus -> ricin poison) is lethal to everything though it's more of a garden plant. Laurel, yes, rhodies yes, laburnum yes but I think it's just the seeds, yew big time. I'm not sure with these if it's just the leaves and seeds or the bark too.
There are also lots of poisonous plants, especially in the garden, but of those in woods, giant hogweed (in that it causes the skin to blister), nightshade, foxglove (but sheep tend to ignore it), ragwort, ivy - in large amounts, otherwise it's a bit of a tonic, oleander, euphorbia. Oak can be a problem if a sheep eats loads of acorns, not because it's poisonous but because it blocks the rumen. For pigs, acorns were a traditional forage.
I think Japanese knotweed is also poisonous, at least to pigs, although they help root it out.
It would be good to find out a definitive list of plants poisonous to all livestock, with an indication of what they do. Anyone good at online research?