Yes, Buttercup is toxic to dogs!
Dogs accidentally consuming the Buttercup 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).
Buttercup consumption is also toxic to Cats and Horses.
Toxic Principles Of Buttercup: Protoanemonin (an irritant).
More Info About The Buttercup Plant
Scientific Name: Ranunculus spp.
Family: Ranunculaceae.
Also known as: Butter Cress, Figwort.
The answer is based on information found on The American Society for the Prevention of Cruelty to Animals (ASPCA) website.