MH Oldschool

yeah the theft function for monsters like gary, melynxes,

Spoiler

chameleos

all only grab items below rarity four (and never bowgun ammo)

I believe Naughty Neko also mentions that they'll never steal anything "too valuable" (I think meaning no monster parts? Unless it's actually tied to rarity), so maybe they're actually doing the economy of Monster Hunter a favor. Without melnyx theft, the whole system would come crashing down in a week.

the market felyne in dos heavily implies he gets a lot of his goods from melynx that have stolen them from hunters, but decides to turn a new leaf as you advance the village

Do we know if they're different genetically? Like are they different species?

We know that they co-habitate, and sometimes melnyx won't attack or attempt to steal items in some areas. They appear to store pilfered items in a communal area, suggesting that felynes may benefit from the theft as well...

how many felynes are just melynx in disguise???