These kill a cat several minutes after the seeds are eaten (As in Forest of Secrets). The flesh helps with a silverthorn sickness (As in The Raging Storm). Deathberries are bright red.
Foxglove seeds can cause paralysis and heart failure (As in Rising Storm). The flowers come in many colors and the seeds are often mistaken for poppy seeds (As in Rising Storm).
These are not as poisonous as other herbs, but are dangerous for kits. They may either kill you or make you sick, the effect is still unknown.
Meadow Saffron is a tall or short plant with white stems and pointed purple petals/flowers. It is not poisonous to prey, but if the prey eats the plant, then the cat consumes the prey, it is deadly. Meadow saffron also causes spasms and seizures (As in Squirrelflight's Hope).
They are used to kill a cat who cannot be saved, quickly. The berries are usually black and shiny when ripe (Nightshade mentioned in Forest of Secrets).
Water Hemlock comes on a variable of green or white flowers in bowl-shaped clusters. It causes writhing, pain, and foaming at the mouth. (As in The Forgotten Warrior).