In US universities, are the sport coaches typically considered tenured professors? The title says it all. In many universities you see coaches for a team in some popular sport hanging around for many decades. I was wondering if in general those people had  a status of tenured professor or equivalent, or if on the contrary they can be "fired at will", if for example the performance of the teams they are in charge of are considered disappointing?

If the answer is really "it depends, there is no rule", then let me say I would like to know what status the Basketball Coach of Brandeis University who has just been fired had (see this Boston Globe article).
No, athletic coaches are generally not considered faculty and do not have tenure.  I've never heard of any place where they are.  

The conditions of their employment would be based on whatever contract they negotiate with the university.  Typically the university would have the right to fire them for any reason, including poor performance of the team, but the contract might call for a severance payment in some cases.

(There could be small schools where someone is a professor and also an athletic coach.  In such cases I'd expect they would have tenure protections in their capacity as professor, but could be removed from the position of athletic coach at the university's discretion.)