I can say in Quran we have:
And the grazing livestock He has created for you; in them is warmth and [numerous] benefits, and from them you eat. Qur'an/16:5
So, eating livestocks are halal, Then:
And [He created] the horses, mules and donkeys for you to ride and [as] adornment. And He creates that which you do not know. Qur'an/16:8
So, Horses, mules and donkeys are different to livestocks in main usage, I can say when those are good for riding so those are not good for eating, but those are still not haraam, Now, I can say those are makruh to eat.
When we tell about eating livestocks it is also about a livestocks's milk.
I can add this when eating a livestock become haraam, drinking its milk will be also haraam.
Some sources add that when a person has sexual intercourse with them those animals become haraam.
Note: AFAIK, In fermented milk products both alcohol and lactic acid may be produced:
Some Islamic scholars do not consider a halal food even if it is made with all halal ingredients but food flavor in which ethyl alcohol was used as a solvent. But other Ulama recommend Halal product made with all Halal ingredients even if food flavors containing ethyl alcohol as a solvent. *source