Updated
If the horses make any sound, the sounds they make are very delayed. (This excludes walking sounds or climbing up and down a block.) I spawned about 15 or more horses and it takes a while to hear them make sounds when idle. (The sound they first make when you spawn them doesn't count because it's not repetitive over time.)

Steps to Reproduce

• Spawn 15 horses (use spawn egg or /summon horse)
• Lead all horses to a post.
• Wait 5 minutes to hear a horse sound. (I heard a sound when 5 minutes was almost up. I waited 5 more minutes and no horse sound ever happened.)
• Wait for the horses to make idle sounds. (The sound they make when you first spawn them doesn't count because it's not repetitive over time.)
• Notice that the sounds you hear from them rarely happen and are delayed over time.

Old
Horses are missing their idle sounds. The only sound they make is when you first spawn them, they are being attacked or they climb down from a block.

Sounds for horses are found here
https://minecraft.gamepedia.com/Sounds.json