Kiko calls it: "ムササビ" (musasabi)
"Musasabi" is Japanese giant flying squirrel (Petaurista leucogenys).
July corrects her: "モモンガ" (momonga).
Momonga is a type of Old World flying squirrels (Pteromys).
Konrad`s Big Japanese-Russian Dictionary gives interpretation in two ways. In concordance with it "モモンガ" can be translated as "Japanese common flying squirrel" (P. volans), or "Japanese dwarf flying squirrel" (P. momonga).
Only common flying squirrels range includes Siberia and Far East of Russia, and it is highly unlikely that somebody would import real mononga from Japan exclusively for Suo. At the same time Mikhail Pavlichenko`s passion with hunt will easily explain the choice of Suo`s pet.
There are two different names of P. volans in Russian and in English. In Russia it usually refers to as the "Common flying squirrel", but elsewhere the term "Siberian flying squirrel" is broadly accepted. The common flying squirrel is the only type of flying squirrel subfamily, that exist at territory of Russia.