A "guided trail ride" is a trail ride where a "guide", or "wrangler", takes you out on the trail. The guide knows the horses, knows the best trails, and helps the riders in controlling their horses.
The wrangler helps you to have a safer and better ride.
A "horse rental" facility rents you the horse to take out
by yourself. The advantages are that if you are an experienced rider you can go
where you want (but you still won't know the good trails) and you can sometimes
get the horses to run (high risk of falling off if you are not an experienced
rider).
When we as horse professionals ride a new horse, we are
very cautious, wearing a helmet and going out with another horse professional
and no novices, until we get to know that horse very well, and get to know that
he will not do anything crazy. So why would a novice want to ride a strange
horse that you don't know without any guide? This is something we do not
recommend. We feel that the horror stories related to the misadventures and
injuries from this practice give horseback riding a bad name.