Brace yourselves. Stupid question incoming.
Not sure how to Google or explain it, so I’ll just use an example. 
“This is the stable where we keep our horses.”
Let’s rephrase that.
“This is our horses’ stable.”
How do we pronounce the word in bold? Is it “horses” or “horses-is”?
Does this apply to words that end with an s, even if it’s not plural? For example: “The bus’ wheel deflated”. Is it pronounced “bus-is”?
Thanks in advance. Now excuse me as I go light my C2 certificate on fire. 
That would depend on context are we talking one horse?
https://www.grammarphobia.com/blog/2011/12/possessive.html
I’m not a native speaker, so this just my opinion. If you use an apostrophe after a word with “s” and leave out the “s” , you don’t pronounce it. In your case it’s “horses”. But if you have for example a name like Jones, which is quite a common name, you can write the “s” after the apostrophe “She is Mrs. Jones’s friend.” and it’s “Mrs. Jones-is friend”. So, it depends on the question, if you write the “s” after the apostrophe, you pronounce it “-is”. No “s” no “-is”.
I’m waiting for the native speakers 
your actually very right
Great link! 
From what I got, the possesive 's is always added (when needed) and fully pronounced, regardless if the word it’s added to ends with an s.
Also, some people use only the apostrophe on words that end with an s but that doesn’t change the pronunciation.
Thanks a lot!