Pour the fresh mare's milk into a clean saucepan. Heat gently over medium heat until it reaches about 90째F (32째C). Do not bring it to a boil.
Remove the milk from heat and allow it to cool to about 70째F (21째C).
Stir in the plain yogurt into the cooled milk. This will help introduce beneficial bacteria to ferment the milk.
Cover the saucepan with a clean cloth or lid and let it sit in a warm, draft-free place for 24-48 hours. The exact time may vary depending on the ambient temperature and the desired sourness.
After the fermentation period, the milk should have thickened and developed a tangy flavor. Stir in sugar and salt if desired.
Chill the Kymyz in the refrigerator before serving.
Enjoy your meal!