Deathberries
Description: Red berries from the dark-leaved, poisonous yew bush.
Location: Unknown
Usage: Sometimes used to kill other creatures by making them eat the berry.
Effect: Kills within minutes when consumed.
Foxglove Seeds
Description: Tiny, black seeds from the bell-shaped flower of the foxglove plant.
Location: Unknown
Usage: They are used to treat the heart
Effect: They can easily cause paralysis and heart failure
Holly Berries
Description: Plant with spiny leaves that produces red berries with no medicinal value.
Location: Unknown
Usage: Unknown
Effect: Not a bother to adults, however can be poisonous for pups and cubs.
Nightshade
Description: Tubular flower with radiating petals
Location: Unknown
Usage: To kill a canine or feline who cannot be saved quickly.
Effect: Poisonous
Water Hemlock
Description: Green or white flowers with petals in umbrella-shaped clusters
Location: Unknown
Usage: Unknown
Effect: Causes writhing and foaming at the mouth.
Since these were taken from the Warrior Cats series, and they are poisonous to cats, it can't be said that all these will kill larger wolves and felines.