If we don't just stick with Mareep (since the design is mostly the same even though the name is different), what about Baampere?
Vind (vine + wind) -> Tangela -> Wrapunzel (wrap + Rapunzel for the focus on the hair, especially as Rapunzel was named for a plant, if not one I think is particularly relevant to the Tangela line)?
Molalone (mola + alone), Ranchor (rancor + anchor) and Groutesque (group + grotesque).
It's not perfectly fitting in meaning (rancor is more lasting negativity than immediate rage), but from the different emotions all of the stages seem to be expressing (sad, angry, smiling), I kind of interpreted it as something of a story: it starts lonely and sad, then that loneliness turns to resentment and bitterness, but then it evolves into a gulper eel and it is super happy to welcome all of its friends back.
Into its mouth.
And stomach.
(Also, Molalone is sort of a reference to sunfish-1, but I think the consensus is that that was probably a placeholder and not its actual intended name, so it's just kind of tongue-in-cheek and not a serious incorporation.)
It's interesting that Crobat was called X-ing, because I always interpreted the cro- as being a play on acrobat, not cross.
Spinarak's old name simply has to be derived from the itsy-bitsy spider. I think Itsybitsy in itself would carry the obvious meaning because that song is basically the only time the term is widely used, but can anyone think of something more punny or clever?
Ariados's name "two-face" brings to mind Janus, but I'm not sure how to work that in. Jaranea? (Janus + aranea?)
But as with Mareep, I think it's better just to keep their canon names since they have them and aren't radically different in design.
For evolved bell cat, its Japanese name being derived from "bell" and "bouncy" mostly makes me think of jingling.
I would also like to propose Straike and Enferno for Raikou and Entei, but can't think of a similar pun for Suicune.
