Beside the ordinary monsters, that can be found almost on every step, you'll encounter their mutated versions. They're stronger, more resistant and have more HP. The main purpose of their existence is just to check them off in bestiary and get the Ba'Gamnan's figure in Sky Pirate's Den. Of course there are also different advantages like leveling up with Neglamuur or Deathbringer from Ithuno.
The ways of finding certain monsters you'll have to confront with other parts of this guide e.g. about opening door somewhere. Usually I place a footnote to that specific section.
When I say: "create a chain" I really say: "Kill these monsters till the digit showing the number of killed monsters starts blinking".
No. 1 - Aspidochelon
Cerobi Steppe - randomly replaces one of Adamantites there.
No. 2 - Thalassinon
Go up the hill in Phon Coast and wait there for a while. You'll see it below.
No. 3 - Greeden
You'll find him on Dalmasca Estersand, on south bank of Nebra.
Kill everything there except for the passive cactiles. After a moment he should appear on that bank.
No. 4 - Bull Croc
It spawns in area on Ozmone Plains called The Shred, actually on its northern part. It appears randomly.
No. 5 - Gavial
He can be found in Garamsythe Waterway.
But to make him appear you have to have the Sluice Gate key (for details check the "Ordinary hunt notices" section for the "White Mousse (rank V)") and close sluices No. 4 and 10. Then go downstairs to the drained canal and go west as far as you can. You'll find some treasures there and in that moment you should be attacked by Gavial, which will jump out from the corridor behind you.
No. 6 - Wary Wolf
You'll find this one in Mosphoran Highwaste in the area called Dalmasca Estersand, but to make him appear you have to create a chain with Worgans.
No. 7 - Kaiser Wolf
Appears in Dalmasca Westersand. To make him appear you have to kill No. 8 - Lindbur Wolf first and without leaving the Westersand come here.
No. 8 - Lindbur Wolf
Can be encountered in Dalmasca Westersand after making a chain from wolves.
No. 9 - Dreadguard
Feywood - create a chain from Mirror Knights. If it didn't appear leave the area and re-enter.
No. 10 - Crypt Bunny
Feywood - kill everything that moves in this area.
No. 11 - Spee
Salikawood - kill all Wyrdhare's in this area and go to the dead end.
No. 12 - Rain Dancer
Giza Plains - rainy season. Sometimes jumps out of a pond west from the south save crystal.
No. 13 - Razorfin
Garamsythe Waterway - kill every Ichthon there. If it doesn't appear leave the area and re-enter.
No. 14 - Apsara
Phon Coast - create a chain from Piranhas (about 10)
No. 15 - Rageclaw
Salikawood - appears from time to time
No. 16 - Binding Mantis
Golmore Jungle - appears from time to time
No. 17 - Wood Toad
Salikawood - appears from time to time. You can easily miss it, because it's under the effect of Vanish. When you find it and it doesn't attack you cast reflect on yourself and then cast a spell on you to destroy his camouflage (do the same thing with to every other half visible monster).
No. 18 - Tarasque
Tchita Uplands - create a chain from Serpents and it'll jump out of water.
No. 19 - Grimalkin
Tchita Uplands - appears randomly, replacing one of the coeurls.
No. 20 - Nekhbet
Estersand - appears after killing some cockatrices loafing around there.
No. 21 - Glaring Eye
Henne Mines II - you have to kill at least three Hectyes
No. 22 - Clutsworn Lich
Tomb of Raithwall - after you activate the south switch Liches start appearing there. Hit them till they start using divide. One of the newly created liches will be the Clustworn Lich.
No. 23 - Evil Spirit
Great Crystal II - you'll certainly meet thin one while on your way to Ultima or Omega XII. It appears on areas with Way Stones on maps IV and V
No. 24 - Juggernaut
Feywood (after the plot reaches this moment) - appears from time to time.
No. 25 - Molen
Zertinan Caverns - go through the hidden area to the area with Grenades. Kill all of them and watch the other bank. Molen should appear in a while.
No. 26 - Tower
Pharos at Ridorana - Third Ascent. The biggest probability of him appearing is if you have chosen to follow the hidden path and killed everything on your way (details in section about that location)
No. 27 - Grey Molter
Mosphoran Highwaste (The shrines must be working) - appears randomly.
No. 28 - Midgardsormr
Golmore Jungle - appears after you have killed everything in the neighborhood. If not then leave the area and re-enter
No. 29 - Nazarnir
Giza Plains - from time to time replaces a Sleipnir there.
No. 30 - Victanir
Nam-Yensa Sandsea - appears from time to time. It's half visible.
No. 31 - Gemhorn
Lhusu Mines II (Site 11 Key is needed) - kill everything in the area including the bugs. He should appear in the dead end. If not just leave the area and re-enter.
No. 32 - Luxollid
Pharos at Ridorana - Umbra. In order to make Luxollid appear you have to kill No. 33 - Vargant Soul.
No. 33 - Vargant Soul
Pharos at Ridorana - Abyssal. He can be found in the northern part but only if you have taken care of the Shadowseer already (see "The Seer (rank VI)")
No. 34 - Imdugud
Nam-Yensa Sandsea - to this area you can get only through Zertinan Caverns. Kill everything that moves in this area, go on that platform and wait till Imdugud appears.
No. 35 - Bull Chocobo
Ogir-Yensa Sandsea - appears from time to time.
No. 36 - Skullash
Phon Coast - appears from time to time on a cliff between Pyrolisks.
No. 37 - Myath
Stilshrine of Miriam - get to this area through the chamber with the save crystal. There you'll find three Dragon Aevises. Kill them, leave the area and re-enter. It should appear in their place.
No. 38 - Helvinek
Necrohol of Nabudis - it suddenly appears while you're fighting with monsters in this area of this Necrohol.
No. 39 - Pallicant
Ridorana Cataract - defeat everything in this area, exit and re-enter. It should be waiting for you there.
No. 40 - Phyllo
Golmore Jungle - it appears from time to time. Most often in that dead end.