How do legendary encounters work?

In practice that means the legendary is not something you stumble into and catch on the spot. The gate is crafting. You build a legendary Anipod first, and crafting it is what unlocks the encounter to battle Irisalis at all. Until that item exists, the fight is not open to you.

Then the order matters. Irisalis is fought in its story encounter and has to be beaten there. Bonding happens after the win, by throwing the crafted Legendary Aniipod at it. So the crafted pod does double duty: it opens the door, and it is the thing you throw at the end. Going in without one means there is nothing to bond with once the fight is over.

Because this is the framework every later legendary is built on, the shape is worth learning once. Craft, unlock, defeat, then throw. If you are hunting for what "legendary Aniimo" means as a system rather than as a single creature, that loop is the answer, and the Irisalis fight is the version of it you can actually point at. Expect the name itself to appear in more than one spelling, including Iris Alice and Erissalis, which is normal for pre-launch material.

Not settled: Only the Irisalis encounter is known in full, so how much each future legendary varies within that template, and whether any of them change the crafting requirement, is not settled yet.