Monastic rules are generally written for purposes of inspiring and governing a community–when and where and how to pray, describing the nature of the ministry of the community, who makes the decisions, who does the dishes, who takes out the garbage, who cares for the sick and the guests, who settles the quarrels….you get the idea.
Historically, many hermits and anchorites have lived under the Rules of the monasteries to which they were attached, with exceptions to the rules for their particular circumstances; others have been directly under the authority of their local bishop. And there have been yet others who were, well, simply solitary. Such persons may or may not have had a formal rule.
In my own situation, while not being attached to any monastery or religious community, but rather making my vows directly to my bishop, I chose to draft my own rule as a contemporary expression of the eremetic tradition. It may be viewed here: Anchorite Rule