Quote Originally Posted by mekko
But are they some special rule about change yxo to уши?
I doubt that you would change the word Mope to Moшe or something.
Besides singular and plural, there was also dual in Old Slavonic.

Thus, "уши" is the old dual form of "yxo" which has always been used instead of the normal plural form, probably "yxа", which is not attested.

Another example is "око" - "очи". These are archaic forms for eye/eyes which are "глаз"/"глаза" in modern Russian. The old plural form of "око" was attested to be "очеса".

In the case of "yxo" (ear) and "око" (eye), the dual forms were used more often than their plural forms and finally replaced them totally.

There is one more influence from the old dual forms into modern Russian: this is the form used after 2/3/4. Modern Russian grammars say Gen. Sing. forms are used that case. Actually, it is reflex of the old dual forms.