Is a holly tree toxic to animals (dogs/cats)?
The ASPCA website on plants which are toxic to animals lists holly (Ilex spp.), as does the Humane Society website.
According to Plant Alert, A Garden Guide for Parents (by Catherine Collins; 2001), and Plants That Poison (by Ervin M. Schmutz and Lucretia Breazeale Hamilton; 1979)
the red or black berries on holly are poisonous to humans as well, and can be fatal to small children if eaten in quantity.
If you believe your dog or cat has consumed holly berries, call your veterinarian for advice as soon as possible, or call the ASPCA Animal Poison Control number, 888-426-4435 (not a free service).