Always worth a look here - http://www.termiaduraddysg.org - especially for technical terms in many fields. The aim of y Termiadur is to standardise terms across Wales for things taught in schools and further and higher education.
If you use the ap Geiriaduron dictionary on an iOS or Android smart phone (the app is free) you can choose to include y Termiadur in its sources of words.
The base word is llygod, which is shorter :)
Mice belong to the category of things which you often find in groups, so the basic word refers to the group, and you use a longer word to refer to a single one of the group.
A bit like children (plant; one child = plentyn) :)