Learn French: L'animal commence le trot. Est-il agile ! - The animal begins the trot. Is he ever agile!

This is a stylistic exclamative inversion. By swapping the subject pronoun (il) and the verb (est) before an adjective, you create a more elegant, slightly literary exclamation. It means the exact same thing as Comme il est agile ! or Qu'est-ce qu'il est agile !

If Est-il agile has the word order of a question, how do people know I am not asking a question when I speak?

Intonation is the key. For a question, your voice usually rises at the end. For an exclamation like this, your voice will typically start high on the verb to emphasize your emotion, and then drop in pitch toward the end of the sentence.

If I wanted to use the noun l'animal in the exclamation instead of il, would I say Est l'animal agile !?

No, just like with standard question inversion in French, you cannot simply invert a noun and a verb. You must use complex inversion: state the noun first, then add the inverted pronoun after the verb. It would become L'animal est-il agile !

Why does the first sentence use the noun le trot instead of the verb phrase à trotter (to trot)?

Both are grammatically correct, but using the noun le trot (the trot) is a stylistic choice often used when talking about animals like horses or dogs. It focuses on the specific gait or stage of movement, treating it as a defined state rather than just an action.