Ah I see — I never thought of simulating it like that. Thank you though!

Yes, unfortunately there is no placeholder feature for galleries. Fandom's older platform used to have one for individual images but that disappeared with the platform update back in 2020-2021.

As for your original question, you should be able to do this using the #tag parser function to call the gallery tag instead of calling the gallery tag directly. That said, using #ifexist like this (or even as in your original attempt) is likely to cause issues if people try to add to the gallery from visual edit mode.

{{#tag: gallery |
File:{{#ifexist:File:Yellow Horse.png|Yellow Horse.png|Placeholder Horses.png}}{{!}}Yellow
File:{{#ifexist:File:Blue Horse.png|Blue Horse.png|Placeholder Horses.png}}{{!}}Blue
File:{{#ifexist:File:Green Horse.png|Green Horse.png|Placeholder Horses.png}}{{!}}Green
}}

Edit: It appears I misunderstood the prior discussion about placeholder images. Even the retired feature isn't really what you are looking for.

You have to create the gallery with the #tag: parser function to be able to use templates or parser functions within it.

{{#tag:gallery|
{{#ifexist: File:Yellow Horse.png|Yellow Horse.png|Placeholder Horses.png}}{{!}}Yellow
{{#ifexist: File:Blue Horse.png|Blue Horse.png|Placeholder Horses.png}}{{!}}Blue
{{#ifexist: File:Green Horse.png|Green Horse.png|Placeholder Horses.png}}{{!}}Green
|widths=250|attribute=value|otherattribute=value}}

Note that using #ifexist creates entries in Special:WantedPages for those images that don't exist, and that it is an "expensive" parser function, meaning it's limited to 100 per page.

/edit:

Wow, 12 min too late. I should reload before hitting save...

No worries — thank you all! I will be checking out each option for the best fit.

Oot42, as you well know, it happens to all of us.🤣

Just to add a bit of clarification to Oot42's comment on #ifexist being an expensive parser function. The limit is 100 expensive parser functions per page including any transcluded by templates used on the page. It is not 100 of each expensive parser function.

Btw, if you want the caption only if the image exists, move it inside the condition:

{{#ifexist: File:Green Horse.png|Green Horse.png{{!}}Green|Placeholder Horses.png}}

Or for different captions:

{{#ifexist: File:Green Horse.png|Green Horse.png{{!}}Green|Placeholder Horses.png{{!}}Placeholder}}

Fandom galleries don't need the "File:" part before the filename, by the way, that's why I removed it in my example.

Andrewds: Yeah, I usually start typing my answer, but then decide to first test or look up something before finally posting it, so it happens quite often.

It's always funny though how posts often have no answers for hours (3 hours in this case), but then suddenly two or more answer at the same time... ;)

Considering it’s an expensive parser function (and I’m assuming you mean it’s limited to 100 per template page), would making another template with more of the gallery, and then adding that new template into the current template code be a workaround of that limitation? Or is it something you should’t do?

I may have read that wrong, but I’m curious anyways —

No, the limit is 100 expensive parser functions per page. Transcluded pages (templates) count the same as functions used directly on the page. You cannot circumvent this by using templates or subtemplates.

There are other expensive parser functions (like {{PAGESINCAT}} ), but not too many, and the limit is 100 for all together.

Alternatively, you could make a DPL gallery. If you can make sure that all uploaded "Horse" images (or whatever images that should appear in the gallery) are always added to a specific category, you can make a gallery that updates itself automatically when new images are added to that category. However, you wouldn't have placeholders than.

For this, the DPL3 extension would have to be activated on the wiki, which has to be requested with Fandom support.

Alrighty, well thanks for the help and clarifications :D