Yes, Hellebore is toxic to dogs!
Dogs accidentally consuming the Hellebore plants can show the following clinical symptoms: Vomiting, Prolonged Depression, Incoordination, Hypersalivaton, Sleepiness Or Excitation, Dilated Pupils, Low Body Temperature, Low Blood Pressure, Coma, Seizure And Death (In Rare Cases).
Hellebore consumption is also toxic to Cats and Horses.
Toxic Principles Of Hellebore: Bufadienolides, glycosides, veratrin and prtoanemonin.
More Info About The Hellebore Plant
Scientific Name: Helleborus niger
Family: Ranunculaceae.
Also known as: Christmas Rose, Lenten Rose, Easter Rose.
The answer is based on information found on The American Society for the Prevention of Cruelty to Animals (ASPCA) website.