We don't know if there have been other Pokémon with the Bond Phenomenon ability, and we don't know if the mutation is exclusive to Kalos. It's just so happens that Kalos is where it has been reported, at least as far as we, the audience, knows.
And there you have it, that's an example of bad writing. If you are going to give one of your characters a special power make sure it doesn't contradict anything in your universe or add limitations to this set power this why Mega evolution is easier to explain that Battle Phenomenon"
Mega evolution is an example of a power up that has clear set rules
1) Strong bond with trainer and Pokemon and trainer
2) Only works with two sets of stones
3) Limited number of pokemon have the ability to mega evolve due to the rarity of the mega stones
4) Mega evolution didn't exist till AZ fired his weapon in Kalos so most stones are in Kalos
Ash-Greninja
1) No mention and no in-universe explanation to why its only limited to the Greninja line
2) Not explanation to why there haven't been other Pokemon that have gained access to this form in the history of Pokemon
3) No mention how the previous Greninja gained access to this form, there was no mention of it having a trainer.
Most of this shows they just made up the form as the anime went along with no clear planning sometimes this works other times is causes problems.
Ash's bonds with his other Pokémon didn't trigger a transformation because none of his other Pokémon possess the Bond Phenomenon ability. We keep saying this over and over, and you keep seeming to miss this: the Bond Phenomenon ability is an ability the Pokémon itself possesses. It's not just about the Pokémon's bond with its Trainer. Strong bond or not, a Pokémon can't transform without the Bond Phenomenon ability. Professor Sycamore made that quite clear in his explanation of the Bond Phenomenon.
Again no I didn't miss it dude...
"Bond Phenomenon ability is an ability
the Pokémon itself possesses" is never confirmed in the anime and nobody even used it as an explanation till the games comes out and as I said the games and anime canons are different and are not the same.
If a character says "X Pokemon has the ability to mega evolve" do you take it as it having a special ability to mega evolve?