In every generation there is one starter family that, while aesthetically awesome is pretty bad in actual gameplay. In the 6th gen it’s the Fennekin line that gets outclassed by its peers. In the 3rd gen it’s the Treeko line that Gamefreak seems to hate.
({fennekin “outclassed” it could destroy chestnaught, and it can learn more the just psychic and fire type attacks as delphox, and treecko is good in sapphire, slightly, })
When I say that a Pokemon is outclassed, I do not mean within the story of the game, but within the metagame. Delphox is outclassed by its peers. Its stats lead it towards a special tank, capable of putting out damage while soaking hits, but it’s fast enough to outspeed many of the competitors in that category. However, its HP is not good at 75, its Defense is terrible at 72, and its Special Defense is only passable at 100. It’s best stat is its Special Attack, but despite being able to learn more than just Fire and Psychic attacks, its special movepool is shallow and aside from its STAB attacks consists almost solely of Grass, Normal, Ghost and Fairy (in ORAS). Delphox can do damage to some Pokemon, but it has an awful time taking hits due to its subpar defenses and HP, as well as weaknesses to some of the most common attacking types. Take hidden abilities into account and Delphox is worse, which is a shame. Its hidden ability Magician is effectively a free Thief, as it takes the item of any Pokemon that makes contact with it, but its Defense is so bad that it cannot afford to take that hit.
Chesnaught has very, very good Defense, and coupled with good HP a passable Special Defense. Meanwhile, it also has really good attack, and access to many of the best physical attacks in the game. Chesnaught can potentially tank a hit from Delphox. Delphox cannot tank a hit from Chesnaught. Chesnaught’s hidden ability outclasses Delphox’s exceptionally as well, with Bulletproof giving it full immunity to many attacks and allowing it to completely wall some adversaries.
Then there’s Greninja which is fast and fragile but with a very good movepool that can be abused from either end of the attacking spectrum without its hidden ability, and especially with its ability of Protean allowing it to change typing on the fly and giving it STAB on every move.
In 3rd gen, Treeko is not good. In the story against Aqua it can be useful, but so can every other Grass type. Its stats make it ideal for a Special sweeper, hitting hard and fast with Special Attacks. However, its movepool is primarily physical and its Special coverage is worse than Delphox’s. Getting a Mega Evolution in its line didn’t help it, especially not in comparison to the others. Mega Sceptile is blindingly fast, and hits hard, but suffers from the same issues as regular Sceptile, it has very, very few moves that it can abuse its high Special Attack with. That its typing gave it 2 more weaknesses, doubled an existing and abusable one, and only removed one weakness and its ability gives it immunity to a type it already has a 4x resistance against doesn’t help matters.