Yes, Hyacinth is toxic to dogs!
Dogs accidentally consuming the Hyacinth 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).
Hyacinth consumption is also toxic to Cats and Horses.
Toxic Principles Of Hyacinth: Possibly narcissus-like alkaloids.
More Info About The Hyacinth Plant
Scientific Name: Hyacinthus orientalis
Family: Liliaceae.
The answer is based on information found on The American Society for the Prevention of Cruelty to Animals (ASPCA) website.