There are at least 9 legendary that can be caught in Pokémon Infinity Island for free just by going through the main story line quests.

• Suicune (Level 30 required)
• Zapdos (Level 32 required)
• Regirock (Level 36 required)
• Raikou (Level 40 required)
• Registeel (Level 50 required)
• Entei (Level 56 required)
• Moltres (Level 65 required)
• Regice (future update)
• Articuno (future update)

Suicune

The first legendary it's going to be Suicune which you'll need to be level 30 in order to find it. Catching this Pokémon in early game it's going to make your journey a brise, from Pewter City all the way to Cerulean City at level 46.

All you have to do is go over the Viridian City Temple which is located righ in the middle of the city so it's quite impossible to miss him.

Zapdos

Before you make your way to the Pewter City Gym and if you're at level 32 you can go catch Zapdos, located at Pewter City Wild Area.

From there, go to the Power Plant Zapdos will be waiting for you on the left side of the building.

Follow the road until you see the Electric type Legendary Bird.

Regirock

Regirock will be available to you at level 36 just by keeping doing the Pewter City's main quests. By doing it so you're going to make your way into Ignis Volcano.

Keep going up the volcano's path and after dealing with the grunts you'll have a change to battle and capture Regirock

Raikou

If you want to catch Raikou you must be at least level 40 after that go to the Cerulean City Wild Area which is located north of the city. Once you are at the Wild Area go over to the big park somewhere in the middlle of the map as shown below, there you'll have the chance to face this legendary.

Quick tip: If you have access to the Hatch fast travel, use it to teleport yourself right into the Cerulean City Wild Area!

Registeel

In order to fight Registeel you'll need to go north of Vermilion City and be at level 50. There you'll find the passage to the Vermilion City Outside area.

Once on the outside of the city keep going north in the direction of The Forgotten Beach.

Finally, you can now make way until The Granite Cave. After you defeat all the grunts inside you'll find Registeel at the end of this cave.

By the way, don't get too excited about the item on the back...it's only a tiny good old Pokéball!

Entei

Being at level 56 means you catch a new legendary Pokémon, Entei. Fly to the Pewter City and go for The Desert Lost on the left side of the city.

Quick tip 1 :  Bring a Pokémon that can change the weather otherwise Entei will die because of the Sandstorm. Something like Rain Dance will do the trick.

Quick tip 2 : You might want to progress a little more with the main story line quests until you reach level 57 because the quests will automatically send you right in front of Entei.

By doing it so, you'll get a little bit stronger just before battle with him. But if you just want to know it's location there's the picture bellow.

Moltres

You are now officially level 65, congratz! This means you can dare to the spooky Lavender Town where lies the mighty powerfull Moltres this Pokémon is one of the most easyest to spot just go all the way down of the city and this beautiful beast will be waiting for your in the middle of the grass.