Yes, Rhubarb is toxic to dogs!
Dogs accidentally consuming the Rhubarb 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).
Rhubarb consumption is also toxic to Cats and Horses.
Toxic Principles Of Rhubarb: Soluble calcium oxalates.
More Info About The Rhubarb Plant
Scientific Name: Rheum rhabarbarium
Family: Polygonaceae.
Also known as: Pie Plant.
The answer is based on information found on The American Society for the Prevention of Cruelty to Animals (ASPCA) website.