not illegal, you can shoot any non-game or non-native species at night, for example, coyote, bobcat, hog, axis, fallow, sika, any ram other than Desert Big Horn (which has other restrictions also.)

You can not shoot animals like whitetail, pronghorn, mule, turkey, dove, quail, duck, geese at night.

This is a very high level summary, there are specific details on the Texas Parks & Wildlife Website.

jason