User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-4639789-20120211213425/@comment-4639789-20120212231629

Could be both. Plesioth do have legs which seem to be evolved for moving on land to some degree, which could support the thoery that it may breathe air. However, it seems to rather stay in water for prolonged periods of time, which could support the theory that it uses it gills.

Personally, I think that Plesioth uses its gills rather than breathe air, as it has (or seems to have) evolved from an aquatic animal of somesort.