Ask Mr. Smarty Plants

The quintessential fast-growing trees are:  1)  Platanus racemosa (California sycamore) and here are photos and more information; and 2)  Populus fremontii (Fremont cottonwood) and here are photos and more information.  There is a male cultivar, 'Nimbus', that doesn't produce the "cotton".  The sycamore is listed on the Fire-Wise Plant Material for Sonoma County list.  Neither of these trees is evergreen.

 These next two trees are fast-growing evergreens for your area:  1)  Lyonothamnus floribundus ssp. aspleniifolius (Fernleaf catalina ironwood) is fast-growing and evergreen.  Here are photos and more information.  2)  Pinus muricata (Bishop pine) and here are photos and more information.

 Here is information about fire effects for three of the trees from the US Forest Service database:

Sycamore:

"Surface fires in the bottomland forests in which sycamore occurs readily
kill saplings and seedlings of all species.  Larger trees are wounded by
fire; fire wounds act as vectors of disease, increasing rot and
decreasing plant vigor."

Cottonwood:

"Mature Fremont cottonwood trees are  top-killed by moderate fire.  The cambium layer is damaged by even low-severity surface fire.  In California, a severe wildland fire completely consumed the understory vegetation of a Fremont cottonwood community. Fremont cottonwoods that were top-killed by the fire were sprouting vigorously from the root crowns." 

Bishop Pine:

"Older trees have thick bark, which enables them to survive surface fire in woodlands and savannas."

Catalina ironwood was not included in the US Forest Service database.

You certainly don't want to plant your tree or trees and have the horses eat all the leaves, rub against the trunk and break it, or eat the bark.  Here is advice from a nursery in Texas about protecting newly planted trees from livestock and wildlife.

It is also important that your tree is not harmful in any way to the horses.   None of the above trees appears as toxic on the ASPCA Toxic and Non-toxic Plant List – Horses.  Trees that should be avoided for horse enclosures are:  Quercus spp. (oaks), Prunus spp. (cherries, chokecherries, peaches, apricots, plums, almonds), Acer spp. (maples) and Pinus ponderosa (Ponderosa pine).

You can check the following toxic databases for plants that could be potentially harmful to horses:

Cornell University's Plants Poisonous to Livestock

Canadian Poisonous Plants Information System

University of Pennsylvania's Poisonous Plants

Toxic Plants of Texas