Ash never had Primeape as a Mankey, yet we mention it because it was a Mankey at one point. Same here, we mention the egg because it was shown to have hatched from an egg. The trivia is about what is revealed and what isn't, all Pokémon hatch from eggs, but Greninja is the first starter to have its egg revealed.
https://bulbapedia.bulbagarden.net/wiki/Talk:Ash%27s_Greninja
Greninja (ゲッコウガ Gekkouga) is a Water/Dark-type Pokémon introduced in Generation VI. It is the final evolution of Froakie, the Water-type starter from Kalos. Greninja, like its previous evolution...
http://pokemon.wikia.com/wiki/Greninja
Main article: Ash's Greninja In the anime, Ash's Greninja is arguably the most powerful Pokemon aside from other Greninja trainers particularly ninjas from the Ninja Village. He has an alternate form known as Ash-Greninja form which it can transforms this form via Bond Phenomenon and its power rivals the power from the Mega Evolved Pokemon.
http://pokemon.wikia.com/wiki/Greninja
Greninja (ゲッコウガ Gekkouga) is a Water/Dark-type Pokémon introduced in Generation VI. It is the final evolution of Froakie, the Water-type starter from Kalos. Greninja, like its previous evolution, has the body style of a frog. In this form, it now resembles Toxicroak and has gained the appearance of a ninja.
http://pokemon.wikia.com/wiki/Greninja
Trivia. 1  Greninja is the fastest of all water-type Pokémon. 2  Greninja learns one of two signature moves: Water Shuriken and Mat Block. 3  It is the only Pokemon with the ability Battle Bond. 4  It shares the same species name as Ninjask, as both are called the Ninja Pokémon.
http://pokemon.wikia.com/wiki/Greninja
Trivia. 1  Greninja is the fastest of all water-type Pokémon. It is also the fastest final evolved starter Pokémon. 2  Greninja learns one of two signature moves: Water Shuriken and Mat Block. 3  It is the only Pokemon with the ability Battle Bond. 4  It shares the same species name as Ninjask, as both are called the Ninja Pokémon.
http://pokemon.wikia.com/wiki/Greninja
Ash's Froakie/Frogadier/Greninja is confirmed to be male because in XY013 he fell in Sylveon's attract, Team Rocket's Meowth also fell in Sylveon's attract comproving that Ash's Greninja is male. - unsigned comment from Pescavelho (talk • contribs)
https://bulbapedia.bulbagarden.net/wiki/Talk:Ash%27s_Greninja
But on Ash's Primeape's and Ash's Krookodile's page it's mentioned in their infobox they had been a Mankey and a Sandile, despite not being Ash's at the time. This is because they have seen as a Mankey and a Sandile. Greninja has been seen as an Egg, so I think it's worth mentioning in the infobox.
https://bulbapedia.bulbagarden.net/wiki/Talk:Ash%27s_Greninja
Main article: Ash's Greninja. In the anime, Ash's Greninja is arguably the most powerful Pokemon aside from other Greninja trainers particularly ninjas from the Ninja Village. He has an alternate form known as Ash-Greninja form which it can transforms this form via Bond Phenomenon and its power rivals the power from the Mega Evolved Pokemon. Ash's Greninja also gains a rivalry with Sawyer's Sceptile and its pre-evolution stage that can evolved into its mega form, Mega Sceptile via Key Stone.
http://pokemon.wikia.com/wiki/Greninja
No, not really. It appears momentarily and was briefly discussed at the end by the ninjas, but I don't think it was anything important. I just think it should have its own section since this is the first time anything like it has happened, not to mention specifically exclusive to Ash's Greninja. As it gets more insight during the episodes Greninja transforms, that's when a section should be made.
https://bulbapedia.bulbagarden.net/wiki/Talk:Ash%27s_Greninja
ash's greninja
Ash's Greninja is arguably the most powerful Pokemon aside from other Greninja trainers particularly ninjas from the Ninja Village.