Yes, Holly is toxic to dogs!
Dogs accidentally consuming the Holly 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).
Holly consumption is also toxic to Cats and Horses.
Toxic Principles Of Holly: saponins.
More Info About The Holly Plant
Scientific Name: Ilex opaca
Family: Aquifoliaceae.
Also known as: English Holly, European Holly, Oregon Holly, Inkberry, Winterberry, American Holly.
The answer is based on information found on The American Society for the Prevention of Cruelty to Animals (ASPCA) website.