Possible Meanings:
* Concentration or Interest: A horse might tilt its ears slightly back when focused on something, like listening to a sound or watching something interesting.
* Mild Anxiety: A horse might have slightly back ears if it's feeling a little anxious or unsure about something. This could be due to a new situation, a loud noise, or even just being around other horses.
* A Sign of Play: Horses sometimes use ear positions to communicate with each other during play. Slightly back ears could signal playful aggression or a playful challenge.
Important Considerations:
* The Rest of the Body: Look at the horse's entire body language. Are they relaxed, tense, or showing other signs of anxiety?
* The Context: What's happening around the horse? Is there something that might be causing it to be nervous or excited?
* The Individual Horse: Horses have different personalities. Some horses might be naturally more sensitive or have more expressive ears than others.
It's best to consider all the clues from the horse's body language, and the context, to get a better understanding of what they are feeling.
If you are concerned about your horse's behavior, it is always best to consult with a veterinarian or experienced horse trainer.