The other lines of the riddle were "decrypted" so I don't need to do those. Which is good as I probably wouldn't have even been able to guess. However, thanks to that, I may have an idea about the last three lines...
Think of his end to start the fun
So we know the one is Cherubi and the sixth is Leafeon. The line before this line says "So take the holder of one" and "the end" refers to evolution in this riddle. In Cherubi's dex entry, it says sunlight makes its berry red and once it uses all its nutrients, it'll evolve. With that in mind, the next line...
For that, you see, ‘s when I sit down
Under the assumption my guess for the last line was right, that means this line is saying the Pokémon either has sunlight or nutrients as an important fact of their evolution. And either the sit down part is literally sitting down or...it could just mean staying still.
My first thought of nutrients was Zweilous while for sunlight it was Sunkern. It was then I found, according to the dex, it only eats morning dew from plants and, specifically its Hoenn dex entires, it stays still so that the nutrients stored in it can be used when it evolves, something it shares with Cherubi's berry used for evolution.
The sixth’s end’s me, I share this crown
And in here, it's saying that it shares its "crown" with Leafeon and under the assumption that this "crown" has something to do with it being a plant-like creature because of the previous lines talking about plant traits and that the one (Cherubi) is the same it (the mystery 'mon), that means Leafeon's "crown" is either that it mainly uses photosynthesis for energy or how it evolves (location based before Gen VIII) because of the last line talking of evolution.
Under the the "crown" of the former, and tying in to the last line, it says in Sunflora's dex entires that it converts sunlight into energy too, as well as evolving from Sunkern which I previously mentioned that according to the dex that it uses the nutrients within itself to evolve and it stays as still as often as it can to avoid using it before then.
Under the "crown" of the latter, and using the previous line as well, the dex entries say Charjabug stays still and stores energy, similar to Cherubi and Sunkern storing nutrients or "energy" and it alao evolves based on location (at least until, like Leafeon later using a Leaf Stone, coincidentally as octoboy mentioned the riddle was made before Gen VIII, later where it uses/can use a Thunder Stone to evolve), which means Vikavolt is also a candidate.
But under the assumption the Pokémon is still plant-based because of the rest of the riddle using plants to lead to the answer...
My guess is Sunflora.
