It means that the person knows A LOT about horses. For example, if you can name all the parts of a horse, then you have horse sense. Generally, it means that the person has good common sense.

Mainly the 6th sense is the extra thin sense a person has or can develope between this world and the extrodinary. A person with a 6th sense can feel the thin vibrations of the unknown or unexplainable. A person who can read blindfolded is an example of the 6th sense.

Nervous system causes the person to sense pain.

I think it could be a person's sense of self. I hope this helps ;)

The last sense a dying person loses is hearing.