Learn French: Que la maltraitance effraie ce chien errant est normal. - That the abuse scares this stray dog is normal.

The phrase Que la maltraitance effraie ce chien errant acts as the subject of the sentence. When a clause starting with Que acts as the subject, its verb must be in the subjunctive. This is because it presents an idea or fact that is about to be evaluated or judged by the main clause (here, being judged as normal).

Effraie looks exactly like the present indicative. How can we be sure it is the subjunctive here?

You are right that for verbs ending in -er, the il/elle/on forms are identical in both the present indicative and present subjunctive. We know it is the subjunctive here strictly because of the grammatical rule requiring it in a subject clause introduced by Que.

Is it common to start a sentence with Que like this in everyday spoken French?

No, it is quite formal and mostly used in writing. In everyday spoken French, you would normally flip the sentence and use a dummy subject: Il est normal que la maltraitance effraie ce chien errant (or C'est normal que...).

Does the word errant only apply to stray animals?

It is the standard word for stray animals (un chien errant, un chat errant), but it comes from the verb errer (to wander, to roam). You can use it for anything that wanders without a clear destination, such as a wandering spirit (un esprit errant) or a knight-errant (un chevalier errant).