This mod allows you to turn into a Hybrid Sim through gameplay interactions instead of just cheating your way through it. It also contains traits to lock out occult forms you don't want to use to prevent Gameplay issues (such as the alien form being deleted). This mod will also stop the add Vampire buff from removing the Plantsim buff.

This requires the XML Injector to apply the traits from the Rewards Store for $0.

To remove all forms:
1) Put your Sim into human form.
2) cheat traits.remove_trait OccultMermaid_Discovered (if your Sim is a Mermaid)
3) cheat traits.equip_trait NoOccultForm_Gauntlet101010

To remove individual forms:

traits.equip_trait traitNoOccultFormMermaid_Gauntlet101010 (remove OccultMermaid_Discovered first)
traits.equip_trait traitNoOccultFormVampire_Gauntlet101010
traits.equip_trait traitNoOccultFormAlien_Gauntlet101010
traits.equip_trait traitNoOccultFormWerewolf_Gauntlet101010

or

traits.equip_trait trait_NoOccultForm_Gauntlet101010 (works for all)

Iced Cream has figured out how to actually stabalize hybrids. Download his mod here.

Some notes:
- This mod allows a Vampire to bite a robot, but another mod is necessary to make a robot a Vampire.
- This mod allows robots to eat harvestable as that's necessary for them to become Plantsims or Mermaids.

Protip: To make a hybrid look like an alien and stabilize it, download MCC and make your Sim start out an alien. Then use MCC to copy the face and body details from the alien form to the human form. Now your Sim looks like an alien in both forms! I found only the eyes didn't carry over using this method.

I don't have time to really check all the options for this, by the way, so yell at me if you find anything wrong.

Pets

This mod edits the werewolf trait to allow animals. However, there isn't really a way to get them through gameplay. Just cheat them in any way you choose.

To feed them use pet bowls or hay (for horses). These will fill any occult motive to the max. Not very immersive, but it's the best I can do.

See also:
Sims 4 Robot Trait Customs - To allow robot hybrids in various flavors. One of the mods found here is necessary for robot hybrids.
Sims 4 Plantsim Trait Edits and Objects - This mod changes Plantsim tuning to allow Plantsim hybrids as well as various other changes to that lifestate.

Fake Traits

These are no longer supported by me and are no longer included.

Hybrid Through Gameplay