I've been talking for years about writing fanfiction, but so far this is all I've ever posted:

https://www.deviantart.com/deadwheat/art/Prologue-Garrett-Griffon-966556573

I've been working on one starring my own species, the wyverns, but I'm struggling with their dialogue- I want them to speak in an old dialect, the kind Luna used in Season Two. You know, "Thy backside is whole and ungobbled", that sort of thing.

I want it to be as good as possible, but I can't figure out what to have them say.

7970195
German is historically a very old language. You could have the wyverns speaking in German and claim it's their universe's equivalent of the Old Ponish language.

7970198
No, sorry, I'm dead set on them speaking English/Ponish in the kind of manner that Luna did in Season Two. Sorry if that wasn't clear enough.

Another example would be the dialogue in the Middle Ages chapter in Live A Live (Spectacular game, that)

The works of shakespeare that haven't had their vocabulary and sentence structure updated to modern standards are probably a good place to look for inspiration. In fact, most renaissance era english literature would be good to look through.

You can go further back in history to middle English from before the renaissance, however it's going to get a bit more difficult to understand with words that are no longer in use and even old characters such as the þ (Thorn). However during this time the style of middle english can vary greatly from author to author due to there being less standardisation in writing, which can make things difficult

Don't feel bad about this. A lot of us have hit roadblocks when it comes to writing fanfics. For me, it also has to do with finding the time these days due to having a lot of other things. Just remember that nobody is rushing you, so you can take all the time you need.