Can I use a hybrid bike tire on my mountain bike? I got a Merida Matts 20 D frame size 18. I want to use a hybrid bike tire. Can I? The current tire is 26*2.1  

Do I also need to change the rim?
So long as the tire diameter (rim size, as stamped on the side of the tire) is the same, and the width is no more than about 30% wider or narrower than the original, and so long as the tire doesn't create interference problems with the frame or brakes, it should be fine.

So in your case you need a tire labeled 26, with a width of roughly 1.3 to 2.8.  But likely if you go much wider than your current tire you'll experience interference problems.

Whether the tire is labeled "hybrid" or "mountain" or even "road" makes no difference. (Though certain types tend to come in certain sizes -- you won't find many 26 inch racing tires, eg.)