Ask Mr. Smarty Plants

It is true that members of the genus Prunus (Plums, Peaches, Apricots, and Cherries) can be toxic. The toxic compounds are cyanogenic glycosides (chemicals that are converted to cyanide or prussic acid when eaten). The compounds are mostly in the leaves and in the seed. Apples (genus Malus) also contain these compounds, but  I had not seen pears (genus Pyrus) being  listed as containing the glycosides until i saw this article from the Kitsap Conservation District in Washington state. The compounds are more concentrated in wilted leaves than in fresh leaves.

I have included a list of databases of toxic plants that will allow you to learn what other plants to look out for.

Cornell University Plants Poisonous to Livestock

Universtiy of Pennsylvania's Poisonous Plants Database

Poisonous Plants of North Carolina

Texas Toxic Plant Database

Additionally, here are databases that are specific for plants poisonous to horses.

Equisearch.com

ASPCA

Ohio State University

To search the lists, I recommend using the scientific name since those names are generally standard, whereas the common names often vary in spelling and usage. 

Since the Lady Bird Johnson Wildflower Center is committed to the care and propagation of plants native not only to North America but to the area in which they are being grown, I would be remiss not to point out that Pyrus calleryana is a non-native plant (introduced from Asia) and is considered an invasive species in some parts of the country.