If someone is a loner, does it mean they have a mental disorder

No a loner is:

A loner is a label for a person who shuns human interaction for a variety of reasons. These reasons can range from a lack of certain social skills (such as people with Asperger's Syndrome), misanthropy, excessive rejection from others, or just for a desire to be with oneself. A common psychological term used to describe the personality of loners is introversion.

Some individuals refuse to interact with others because of perceived or accurate superiority. They wish to only relate to individuals they consider worthy of their time and attention. Therefore, a loner will have very few intimate relationships. Many feel anxious in their presence because they perceive the loner's disdain towards them. It is also common for people to believe them to be arrogant and egotistical.

Related to the above is the socializing aspect of a loner: focusing on certain traits, some loners socialize greater with people with whom they can confide in. Unfortunately it takes time for this bond to occur and when someone unknown to a loner enters the social group, the loner will automatically shell up. Seeming shy or insecure, most loners can only socialize with people they see constantly. This is in part due to the fact that many loners are overly self-conscious and believe people are constantly sizing up their attributes.

While expressing a desire to be alone most of the time, loners do not necessarily shun human interaction completely. An example would be the person who shuns any social interaction with work colleagues beyond what is necessary for fulfilling his or her job description (mainly for practical reasons and to avoid further complicating one鈥檚 professional relationships) but who is actually fun and extroverted during parties or social gatherings with people outside work. This can be observed in certain central European cultures (e.g., Germany, Austria) which actually discourage relating informally in business or office settings.