Yes, Begonia is toxic to dogs!
Dogs accidentally consuming the Begonia 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).
Begonia consumption is also toxic to Cats and Horses.
Toxic Principles Of Begonia: Soluble calcium oxalates.
More Info About The Begonia Plant
Scientific Name: Begonia spp.
Family: Begoniaceae.
Also known as: Over 1,000 species and 10,000 hybrids.
The answer is based on information found on The American Society for the Prevention of Cruelty to Animals (ASPCA) website.