On the latest version, this menu here is an in-game sprite gallery. It has all the animation, but It still may not satisfy people, and it kinda hard to browse through all the equipment.
If it doesn't appear in yours:
1. Is your game the latest version?
2. Have you beat the game yet?
The Monstergirl Unit can only equip one kind of clothes unique to them, Example: Harpy wore Breast-Shirt, HoneyBee wore Tanktop.
But the Human Unit can equip other than their unique clothes type, some even share them,
Example: Blue unique clothes is the Armored-Princess-Gown, but she can also wear the Roman Dress and the Bikini Armor, and the Roman Dress is also equippable to Yellow while the Bikini Armor is also equippable to Red.
Answering your question, there are two ways of getting new Monstergirl Units (Human Unit acquisition is story-driven):
1. InterBreed Birth:
Finish the stage with any of your unit
defeat-by-raped by enemy. This will get you the respective Monstergirl of the Monster raped your units.
Example: Blue Princess KO-raped by Hornet, Slimegirl KO-raped by SlimeGoop, when you finish the stage, you get new HoneyBee and Slimegirl Unit.
Tips: Human Unit can be raped by anything. Monstergirl can be raped by their own species counterpart.
2. InterHybrid Birth:
Other than their own species counterpart, each Monstergirl Unit can also be raped by (mostly)
one of other monster species, this will instigate the birth of an
Hybrid Monstergirl Unit (There are cases though were Hybridisation didn't happen, Ex: Orcgirl with SlimeGoop). Obviously, Hybrid Unit are overpowered compared to normal Monstergirl Unit (But well, IMO the Lamia is the most OP unit even though she's not Hybrid Unit) .
Example: Let HoneyBee be raped by Spider or Arachnee be raped by Hornet, guess what's the Hybrid will be?

Tips: Hybrid unit can be raped by their respective origin species counterpart, this result in that same Hybrid Unit, easy way to duplicate Hybrid, not the way to get all different element though. (Remember, the element of the Unit is decided by their Mother's Element, Blue Princess will always give birth to Water-Elemented Units)
Random info:
Technically, the Monstergirl Unit is a hybrid themself, between Human and Monster, so that makes the Hybrid Monstergirl Unit to be actually a Tribrid.
