I have two different tires in the rear than I do in the back. Not too odd. I'm not sure about only 1 different tire. Is the tread fairly similar? Does the car drift to one side?
If you haven’t noticed anything odd about it's performance or handling, then I guess you should be all right, just be sure the oddball tire is similar enough to the rest of them and you ought to be all right.
Check your spare tire. That may be the tire that had an untimely demise by a large sharp object into a sidewall (or something) and was replaced with the actual spare tire and the person who sold the car to you might not have told you. If I'm correct, and you don't have custom wheels, you should have a spare wheel just like the rest of your wheels, so running on a spare is unnoticeable and normal… except for maybe a mismatched tire brand. …Just a possible theory.