This trait makes it so horses will not woohoo with other horses when sims encourage them to breed.
If you add a ton of new traits for horses to the game, it will add a scrollbar to the UI.

As far as I know, this was the first downloadable modded trait for horses.

And something even this basic was a nightmare to make. Who knows the person who makes Mod Constructor? Can you go tap them on the shoulder? Lol.

If you want 3 normal personality traits + infertile as a fourth, or just want to add this to an existing gelding, you can do so with MCCC.

I just realized "Ho-Hum" was in this screenshot also.! Oops! Ha ha ha. The only reason I can't release that one is I can't get the buffs to work for some reason. It should be a super simple process but no matter what I do, sims that interact with the horse do not receive the buffs they need which would add pie menu options. (Then custom animations made for the interactions.) I wanted to make a ton of fairly in depth horse traits with custom sounds/animations that isn't just barebones 'learns this faster, gets tired slower' but it's really hard to do without Mod Constructor updating. All my manual entries aren't showing in game and I'm doing this alone so it's just hard.

"How?" Tech Info

If you have never made custom traits for the sims, do not start with Horses. It will feel like being ran over and then tweeted about. It's a pretty horrible experience.
If you HAVE made custom traits for sims and know all about broadcasters, interactions, and custom animations, then all you need to do is (on your Sim Data entry in TS4 Studio after exporting the package from Mod Constructor) change 'Species' to 6, and Ages to 4, 32, 64 and the trait will show up for horses instead of sims. :)

The problem and what makes it so much harder for horses instead of sims is there isn't much to work with in the game. It's not like making 'introverted' then making the sims never autonomously do social things or have a high social need drop. Horses are pretty bare bones in terms of things they can even do in the game and they don't even have a proper dirty/clean hygiene system. Anything you want to add will basically have to be done/added yourself. And then certain things require sims to interact with them, which is a little tougher than slapping together some built in value sliders. Since Mod Constructor isn't updated for horses as of writing this, you also have to either create custom entries or paste values discovered in studio, over to the 'custom' section in mod constructor which is pretty simple. Just look at the S=(Numbers) and paste the numbers to the 'custom' field, done. But if that's new to you in general it will feel like moving mountains.

TLDR: Export from Mod Constructor as a package, open the package in Studio, fix all of the entries to reference horse content instead of sim content.
If you are REALLY good at making traits but need someone to make custom animations and CC for horses to go with your traits, PM me and I'll co-op with you. (Only for horse related content.)

[PW] Infertility Trait for Horses