Naroom Hyrens:
On your way to the Underneath, dig at the first path to the left, go down the stairs, next to the guy there's a crack in the wall, put a blast urn there, and go through and follow the path until you reach a secret forest.

Underneath Hyrens:
In Cald, in the cave above Valkan's house, go to one of the stairs and go down. Wait for a swirly teleporter to appear, go into that and you will fight the Underneath Hyrens.

Cald Hyrens:
Right outside the cave that joins Cald/Naroom, go left when you're hitting one of the mountains. You'll get into a secret cave where you fight the hyrens. Beware though, you can't get Flame Hyren until you get some Ayebaw animite from the Arderial Paths.

Orothe Hyrens:
With the Orothean Belt, swim outside the seer's house. Go north-west till you see a dark spot. Go into it and you'll be taken to a sea cave where you fight them.

Arderial Hyrens:
They have been left out of the game. Don't fret though, there are some rumors... notice that on part of the Arderial parts, there is a place where you walk onto the yellow stuff off one of the platforms (don't take a running start, or you'll fly off), use a blast urn... that may work. Other rumors have started at gamefaqs and magi-world that if you fight Orathan F several times then go into the castle, they will randomly appear... none of this is confirmed, so hold your horses!

Core Hyren:
You find them in the 5th Shadow Geyser but you can't get them until the second time through the game (see Secret Start) from which point you go to where the note was and you find the Core Ringsmith instead.

Secret Start:
Save 2 keys from the 5th geyser, and use one from the stone circle under Gruk's house to find a note. Then when you beat the game and after the credits roll, kick the lower-left block on the start/continue screen and go down, dig past the dirt, and open the door. A guy will open the middle door which lets you start a new game with your current Relics, Dream Creatures (at their current level) and Tony at his current level.

Orothan F(lyer)
In Arderial, walk around the mountains just east of the palace, stick close and try inbetween, that's where they appear the most.

Megathan
In the ruins in Orothe (not the vault place, the area near Oscent Mar), use blast urn anywhere and they'll pop up.

Platheus, Sphor, Xyx
They were also left out of the game, Xyx may appear with the Arderial hyrens though.

Ormagon (submitted by whizkid)

there is one thing he missed: you must let orwin die

1) See the Ormagon Hunter after losing to Morag, before seeing Gia.

2) Get the algae and give it to him BEFORE getting Agadon's Boots

3) Run around the statue 5 times counterclockwise in Yaki's Training Ground and talk to the left eye to open to door and get the Fungicide. (BEFORE beating the 3rd Core Gate)

4) In Oscent Mar, get a Specimen Jar.

5) Return to the path to Gruk's house, take a fungus sample, it becomes the JarofFungus.

6) Give it to the boy in Vash Naroom who "likes to collect spooky plants"

7) Beat Core Gate 5

8) Return to Vash Naroom, the house will have been overtaken by fungus. Use the fungicide.

9) The fungus is killed. Take the hollow stool.

10) Return to the path near Gruk's house, where the fungus is growing.

11) Use the fungicide on the wall, it breaks open

12) In the fungus-covered area, use the Hollow Stool.

13) Tony will blow through the legs of the stool.

14) BOOM! BOOM! The screen starts shaking... Tony says "Uh oh..."

15) Ormagon attacks. Good luck. You'll need it.