ITRTG D4 pet options

Staple - Almost always in team
Option - Sometimes in team
Alternative - Strong, but either available so late you'll need to break in with something else, or such a minor upgrade you may as well stick with the obvious alternative, or preferred elsewhere like RTI

Wildcard options - Chameleon, Spoon, Gray (recommended only in an emergency because of missed growth)

1 rogue per team is generally suggested for a balance between power and loot, but any team can go 0 or 2 to sacrifice one for the other

Scrapyard: Defender, Support, Rogue, 3 dps (most commonly Assassin, Assassin, Blacksmith)
Staple - Dog, Tanuki, Carno
Option - Anything, Cat (staple later with improve), Mimic, Archer, Ghost, Taco, Any blacksmith
Alternative - Elemental (late unlock, also the strongest crafter; staple at high difficulties), Strategist (late unlock; often SR HP slot, staple if not), Fool (late evo, typically SR defence slot), BHC (works and solves nanobots, but requires significant investment to manage Railgun with mages' 40% HP)

Water Temple: Witch, Defender, 3 Support, Rogue
Staple - Witch, Mist Sphere, Alien (as support), Clam (as support if also using Undine)
Option - Any defender, Any supporter
Alternative - Leviathan (late unlock), Undine (potentially RTI; staple for ancient mimics)

Volcano: Difficulty 0 some standard team with Vesuvius mage, freeform speed damage based from difficulty 1 or 2 onwards
Staple - Vesuvius, Vermilion Pheasant, Ape
Option - Any rogue, Any support, Book, Balrog, Elephant (early diffs only), Vampire (eventually staple with improve)
Alternative - Salamander (typically RTI), Succubus (potentially SR speed slot)

Mountain: Sylph, 2 Defender, 2 Support, Rogue
Staple - Sylph, Cherub, Valkyrie, Fairy, Catbox
Option - Hourglass, Any support, Any rogue
Alternative -

Forest: 1-2 Defender, 1-2 Supporter, 2 dps, Rogue
Staple - Gnome, Coin
Option - Any defender, Any support, Goat, Hwangeum Pig, Rabbit, Spoon (staple initially, typically moved elsewhere later)
Alternative - Basilisk (late unlock, typically RTI), Crocodile (late unlock, often RTI), Honey Badger (late evo, can be assassin or supporter)