Possible Clues:

• Haggard chap with singular description of horse without jockey
• With no one up slide, errs foolishly
• With no one in the saddle
• Haggard fellow's description of horse in stable?
• Like a bare-backed horse
• Like one running when seat is empty, unqualified?
• Horse thus unencumbered is to make fewer trips across river
• Arabs running wild may be this
• Some Grand National runners may become so out of condition?
• Horse that can cause problems