[Seventh Sea]

Maybe for the ferroseed guess "steal the greener side" could be a reference to steel type and grass type? Not my guess though...

I'd say hoppip.

Spinning these things around and around.
"These things" refer to its leaves, which spin in pokemon mystery dungeon.

They hardly ever touch the ground.
They, the the leaves, almost never touch the ground, and neither do the hoppips, because they are carried around by the wind.

I may be a pain in your side.
For I steal the greener side.
Many of its moves are centered around stealing HP - leech seed, absorb, giga drain, etc.