This guide is organized by emotion for quick in-character reference. Much of this information is thanks to this article on HorseChannel.com as well as various awesome diagrams across the interwebs.

Afraid/Anxious/Nervous

Ears: Rapidly swiveling around.
Head/Neck: Typically held high, muscles tensed. Mouth and teeth may be tense, might grind teeth. Nostrils may be flared.
Eyes: Whites of eyes may be showing.
Tail: Tucked tightly against hindquarters.
Overall: Might be quivering/trembling, often can't stand still/will try to bolt.

Alert/Focused

Ears: Pointed in the direction where the horse is looking.
Head/Neck: Held up.
Eyes: Bright and attentive.
Tail: May be elevated above the back.
Overall: Standing up straight, tense leg muscles.

Content/Relaxed

Ears: Forward or neutral.
Head/Neck: Slightly lowered. Relaxed mouth.
Eyes: Soft, normal blinking, relaxed muscles.
Tail: Relaxed, may swish gently back and forth.
Overall: Generally relaxed. May cock one hind leg.

Annoyed/Stressed/Aggression

Ears: Pinned back, sometimes flat to the skull.
Head/Neck: May grind teeth, toss/fling head. May have pinched nostrils, breathing in hard.
Eyes: Hard, angry look.
Tail: Swishing around rapidly.
Overall: Stiff and defensive. May swing hindquarters at other horse, may give a warning kick.