For the second question, it's easier to link: http://monsterhunter.wikia.com/wiki/Leviathan
The Monster Hunter universe indeed has it's own (simple) biological classifications. <3 flying wyverns
... 'Cept the Gigginox.
And are there any non-third gen monsters that appear in MH3U?
EDIT: Ninja'd twice, not surprised. 'Karp's link was more a bit explanatory, as I thought Plesioths were part of the Lagiacrus for having the same theme.