Poisonous/ harmful plants that I know of:
Buttercups
Rhododendrons and azaleas
Hygdrangea
Foxglove
Bamboo
Jasmine
Horse chestnut
Lilac
Anemone
Eucalyptus
All alliums
All plants that grow from a bulb e.g. daffodil, crocus, tulip etc (although my GPs demolished a gladiolus bed once with no ill effect)
Deadly nightshade and henbane (also leaves of any other nightshade plant though the fruit of tomato and sweet peppers are fine)
Rhubarb
All citrus
Ferns
Horsetails
All spurges
Hemlock
Oleander
Milkweed
Lily of the valley
Yew
Cowslips
Toadflax
Euphorbia
Celandine
All bindweeds
Ivy
Privet
Elder
Holly
Bleeding heart plant
Nettles (can be fed dry but avoid fresh for obvious reasons)
Dock
Any fruit seeds which cyanide e.g. apple pips
Aconite
Poppy
Ragwort
Cautions:
Spinach, beet greens, sorrel, chard etc - they can eat them but limit intake due to oxalates
Apple - too much can cause lip blistering but they usually stop eating it before this occurs
Iceberg lettuce - very watery and can cause diarrhoea
Guinea Lynx used to have a comprehensive database but I can’t seem to locate it right now.