There are dozens of enemies you can use to your own advantage in Chrono Cross. The first way you can use them is via the character Sprigg. Her special technique, Doppelgang, allows you to change her into a monster. The second way is the Grand Slam inside the S.S. Zelbess, where you can pit your monsters against Janice's. If Janice joins your party (i.e. you beat her three times straight) you can't access the Grand Slam anymore.
You start with a limited list of monsters. To make your list grow, you have to defeat more monsters. But there are rules: 1) No monster will be added to your list unless it is defeated by either Sprigg or a character wearing the Forget-me-not Pot accessory. 2) If you're using Sprigg to defeat the monsters, she must be in her normal form. 3) Not every monster is morphable; some can never be added to the list.
There are two major differences between Sprigg's monsters and the ones in the Grand Slam. Each monster in the Grand Slam has its own HP, whereas Sprigg's morphs retain her own HP. However, the monsters in the Grand Slam cannot use ANY L.1 Elements.
The color of the monster name indicates its innate color, as follows: White, Black, Red, Blue, Green, Yellow.
Any Elements with a in front of them are special techs; they're special to every monster (although there are a few repeats). Special techs are always the same color Element as the monster's innate Element, with a few exceptions: the Porre PVT and Porre SGT use Red techs even though they're innately White; Prehysterical uses Red techs even though it's Yellow; and Tragedienne uses several colors of techs.
I've listed all the Element grids for the monsters. There's an extra L.1 Green tech, to the bottom of the entire grid, called Doppelgang; this allows you to change back to Sprigg or to any other monster during battle. Obviously it doesn't show up during Grand Slam (since you can't use L.1 techs then).
If you've missed a monster, you can usually go back to the Bend of Time and fight them there. However, for some monsters, once you've missed them that's it. I've marked those monsters with a (B).
As a rule, the later monsters are stronger than earlier ones. Just because a monster has a large Element grid doesn't mean it's a strong monster. However, I have no accurate method of rating them in terms of strength or magic strength, sorry...
Finally: Most monster attacks are just "damage / one enemy", in the Element of that monster. The few attacks with special effects (status or multi-target) are listed at the bottom of the page.
Acacia PVT Location: Viper Manor (B)
Acacia SGT Location: Viper Manor (B)
Aero-Guard Location: Chronopolis
Airframe Location: Isle of the Damned
Alphabat Location: Fort Dragonia
Beach Bum Location: Lizard Rock
Beeba Location: Hydra Marsh (B)
Bubba Dingo Location: Fossil Valley
Bulb Location: Shadow Forest
Cassowary Location: Shadow Forest
CatBurglar Location: Mount Pyre
Combat Location: Fort Dragonia
Combot Location: Chronopolis
Crossbones Location: S.S. Invincible
Cuscus Location: Shadow Forest
Cybot Location: Fort Dragonia
DaffyDwarf Location: Hydra Marsh, Water Dragon Isle
DaggyDwarf Location: Hydra Marsh, Water Dragon Isle
Dead Beat Location: S.S. Invincible, Isle of the Damned
Dodo Location: Fossil Valley
Drongo Location: Fossil Valley
Dwarf Location: Hydra Marsh, Water Dragon Isle
Flea (absorbs Red Element attacks) Location: Bend of Time (B)
Fossicker Location: Earth Dragon Isle
Gerridae Location: Shadow Forest
Gizmotoid Location: Chronopolis
Gloop Location: Shadow Forest
Gobledygook Location: Shadow Forest
Googhoul Location: Fort Dragonia
Gremlin Location: Dead Sea
Gurgoyle Location: Fort Dragonia
Gyroblade Location: Chronopolis
HotDoggity Location: Mount Pyre
KomodoPup Location: Lizard Rock
Lagoonate Location: Marbule, Water Dragon Isle
Mama Dingo Location: Fossil Valley
Man-At-Arms Location: Viper Manor
Man-Of-War Location: Viper Manor
Mantarrey Location: Water Dragon Isle, Marbule
Myxomocete Location: Terra Tower
Neo-N-Bulb Location: Viper Manor (B)
Opah Fish Location: Lizard Rock
Ozzie (absorbs Green Element attacks) Location: Bend of Time (B)
Paper Boy Location: Fort Dragonia
Porre PVT Location: Viper Manor (B)
Porre SGT Location: Viper Manor (B)
Portalgheist Location: Viper Manor (B)
Prehysteric Location: Gaea's Navel
Preymantis Location: Gaea's Navel
Puffy Location: El Nido Triangle
RockRoach Location: Earth Dragon Isle
SandSquirt Location: Lizard Rock
Scorpiod Location: Water Dragon Isle
SideSteppa Location: El Nido Triangle, Water Dragon Isle
Slash (absorbs Blue Element attacks) Location: Bend of Time (B)
If Sprigg is in a party with Serge and Kid, her Slash morph can use its DashSlash to execute the Blue-Element "Z-Slash" triple tech with Serge's FlyingArrow (L.7) and Kid's RedPin (L.5) techniques.
SnibGoblin Location: Hydra Marsh
SnobGoblin Location: Hydra Marsh
SpearFisher Location: Viper Manor Sewers
Taurminator Location: Mount Pyre
TotalChaos Location: Dimension Vortex
Tragedienne Location: Dead Sea
Tutanshaman Location: Fossil Valley
Tzetze Fly Location: S.S. Invincible
Whoot Location: Terra Tower
WightKnight Location: H Isle of the Damned (B)
WillO'Wisp Location: Viper Manor
Wingapede Location: Hydra Marsh (B)
Witchetty Location: Viper Manor Sewers
Wraith Location: S.S. Zelbess, Shadow Forest
YellowBelly Location: Earth Dragon Isle
I have not tested the following monsters for morphability: BlueMoaman, Criosphinx, Ketchop, KingMoaman, Red Moaman.