Mongolia is buying five MiG-29 fighters from Russia. Mongolia does not need MiG-29s, since it is surrounded by Russia and China, two much more powerful nations. Something else is going on here, and it probably has more to do with keeping the MiG-29 production line going than it does with Mongolian defense needs.
The MiG-29 is not a popular aircraft. Many nations (Algeria, Malaysia, Lebanon) are refusing, or retiring, MiG-29s. One unique exception is Syria, which was eager to get some more of them, at least it was last year. That was because Syria is broke, and patron Iran was becoming less generous (because of its own economic problems) with subsidies for military equipment. Thus Russia announced last year that it was selling another 24 (or more) MiG-29s to Syria (which already has about fifty of them). Syria would like to get its existing MiG-29s upgraded, but may not be able to afford that. But now Syria is undergoing a revolution, and the new government (or even the current one, if it survives), may not want to buy jet fighters no one else wants.
Other nations are backing away from MiG-29s because of reliability and durability problems. Several times in the last few years, Russia has had all MiG-29s grounded because of crashes, and suspicion that there might be some kind of fundamental design flaw. There had been several problems with MiG-29s earlier, although all aircraft were eventually returned to flight status. This has not helped sales, and most export customers prefer the larger Su-27 (and its derivatives like the Su-30).
The MiG-29 entered Russian service in 1983, as the answer to the American F-16. Some 1,600 MiG-29s have been produced so far, with most (about 900) exported. The biggest customer, India, received its first MiG-29s in 1986, with deliveries continuing into the 1990s. The 22 ton aircraft is, indeed, roughly comparable to the F-16, but it depends a lot on which version of either aircraft you are talking about. Then there are the reliability problems. Compared to Western aircraft, like the F-16, the MiG-29 is available for action about two thirds as often.
Syria has not been able to afford to let its pilots spend much time in the air, which reduces wear and tear on the MiG-29s and makes them last longer. Thus the Syrian MiG-29s can expect to provide target practice for more experienced Israeli pilots, flying advanced models of the F-16.
Mongolia is more broke than Syria. In the 1970s and 80s, Mongolia bought 44 MiG-21s from Russia. These were not flown much, and not a lot was spent on maintenance. Currently, only about ten of these MiG-21s are flyable.
