Day/Night
Squisher/Nightsquish
Fanged Squisher/Vampire Squisher
Petunia/Nightshade
Spiked Squisher/Spiked Squisher
Shagdaw/Hellshag (Fire-elemental)
Ground Spitter/Ground Spitter
Swamp Crab/Bog Monster
Armadillo/Armadillo
Dumtilian/Dumtilian and Dire Rat
Agent/Agent
Goo Monster/Goo Monster
Nothing/Mini-Crawler
This is gonna be a tizzle. Anyway, for the nocturnal monsters. We got the Nightsquish, which would replace Squishers in spawners.:

Pretty much the most basic nocturnal monster: a Squisher with big glowing eyes. For its stats, this is what I got.:
HP: 6
Defense: 1
Power: 3
EXP: 2 (Assuming √(HP/4+Power²+Defense²)*0.75)
Drops: Same drops and drop rates as Squisher plus Yellow T Shirt
Pretty basic stuff. Now, for the other guys...

The vampire squisher would replace the Fanged Squisher at night. For its stats, here's what I got.:
HP: 12
Defense: 3
Power: 6
EXP: 5 (Assuming √(HP/4+Power²+Defense²)*0.75)
Drops: Same drops and drop rates as Fanged Squisher plus Suit

The Nighshade would replace the Petunia in Petunia spawns at night. For its stats, here they are.:
HP: 54
Defense: 5
Power: 13
EXP: 10 (Assuming √(HP/4+Power²+Defense²)*0.75)
Drops: Blueberries (Common), Twig (Uncommon), Suit (Uncommon), Dress Pants (Uncommon), Shades (Uncommon)

Yeah, a Fire-elemental monster: the Hellshag. I was thinking about Hellhounds with this guy. Anyway...
HP: 60
Defense: 10
Power: 18
EXP: 15 (Assuming √(HP/4+Power²+Defense²)*0.75)
Drops: Blueberries, Bullets, Fairy Wand, Shepherd's Crook, Grey Wizard Hat


The Bog Monster. Yeah, might as well try something for a Swamp Crab replacement at night. Here are its stats.:
HP: 100
Defense: 25
Power: 25
EXP: 26 (Assuming √(HP/4+Power²+Defense²)*0.75)
Drops: Blueberries, Cherries, Pilgrim Hat, Pilgrim Shirt, Pilgrim Pants, Machete

People really liked the Dire Rat in an ironic way, so why not have it in as a nocturnal monster co-existing with its prey: Dumtilians. Here are its stats:
HP: 160
Defense: 35
Power: 45
Spawn Chance: 20% at Dumtilian spawners at night
EXP: 43 (Assuming √(HP/4+Power²+Defense²)*0.75)
Drops: Milk, Ninja Hood, Ninja Gi, Ninja Pants, Ninja Socks, Kunai, Katana
Now for the last one: a monster that doesn't take up spawners and only appears in night:

Yeah, it's a spider. With poison damage. What else is new?
But yeah, there's that.