How do you longline a horse?
-Depends where you are and what you want to accomplish.
What equipment is needed?
-Depends where you are and what you want to accomplish. Halter, lead, saddle, bridle, training sursingle, training rig, longe whip, dressage whip, "carrot stick" ect. Are all tools used for different purposes.
Should you do a small amount of longlining before each ride to see how the horses behavior is?
-If you want, IMO you cannot gauge a horses "mood" just from longing... most people do it because they are somewhat afraid of their horse, if the horse acts good on a line they ride, if not they don't. Some people also longe for a "warmup" to get the horse ready for under saddle work or to release pent up energy from inappropriate stalling.
Do you enjoy longlining?
-Not really. I personally find it unnecessary when I can do the same thing under saddle.
Does it benefit to the horse?
-In some cases, when done properly, not when you are just driving the horse around in a circle or chasing it around a pen or arena with whip...
It can be very detrimental to a horses muscular and skeletal structures if done wrong (i.e. In too small an area, wrong footing, too often for too long a period of time, in only one direction or primarily one direction, ect. The list goes on and on.)