## 1 Introduction  
Fermented dairy products play a key role in the modern consumer's diet. Awareness of the importance of microflora in maintaining good health is steadily increasing [1, 2, 3], which has led to heightened interest and substantial commercial investment in this area. Consequently, there is a growing demand for functional dairy products in consumers' regular diets. However, cow's milk can trigger allergic reactions in a significant number of individuals, presenting a considerable challenge, particularly for children [4, 5]. To address this issue and expand the range of functional food products available, it is essential to explore alternative food matrices within the dairy market [6, 7]. The use of mare's milk as a functional food has been a subject of discussion for decades. Mare's milk is recognized for its distinctive nutritional profile, closely resemblinghuman milk. This similarity provides potential benefits for individuals with cow's milk intolerance, particularly infants and children. Mare's milk is rich in valuable nutrients and is highly digestible, making it an appropriate substitute for bovine milk in pediatric diets [8; 9]. Research on the incorporation of probiotic ingredients into mare's milk-based dairy products remains limited, highlighting significant opportunities for further scientific exploration and commercial development. Owing to its high lactose content, mare's milk is particularly suitable for producing fermented milk beverages. Furthermore, it is a rich source of essential amino acids and contains a higher proportion of whey proteins compared to cow's milk [10]. Mare's milk has shown potential in managing cow's milk allergy due to its low allergenicity, making it a viable dietary alternative for individuals allergic to cow's milk [11]. The whey protein composition and profile of mare's milk make it more favorable for human nutrition compared to cow's and sheep's milk [4]. Additionally, when consumed in fermented forms, mare's milk may offer enhanced therapeutic benefits [12; 13]. Studies indicate that fermented milk beverages made from mare's milk are characterized by a slightly acidic, yeasts, and astringent flavor profile and are highly valued for their nutritional benefits. The traditional fermented milk beverage produced from mare's milk is kumis, obtained through the combined activity of lactic acid bacteria (LAB) and yeasts [14]. Kumis originates from the nomadic cultures of Central Asia and is widely consumed in Mongolia, Kazakhstan, Kyrgyzstan, Uzbekistan, Turkey, and certain regions of Russia [15; 16]. It is also consumed in Eastern Europe, particularly in Hungary, where it has historically been referred to as \"milky wine\". Like kefir, kumis is characterized by significant carbonation and a low ethyl alcohol content (\(\sim\)2.5%) [15; 17]. The mixed microflora involved in its fermentation includes lactic acid bacteria (primarily _Lactobacillus_ spp., _Lactococcus_ spp., _Enterococcus_ spp., _Streptococcus_ spp., _Leuconostac_ spp. ), as well as fermenting and non-fermenting yeasts (_Candida, Kluyvernomyces_, _Saccharomyces_, _Torula_) [14; 18; 19; 20; 21]. The primary objective of this research is to explore mare's milk containing probiotic bacteria as a potential alternative food matrix to produce functional dairy products. This study investigated mare's milk fermented with a bacterial culture comprising _Lactobacillus acidophilus_ LA-5 (LA-5) and _Bifidobacterium animalis_ subsp. _lactis_ BB-12 (BB-12). These bacteria are naturally present in the human digestive system. However, factors such as antibiotic treatments, aging, stress, and poor dietary habits can disrupt the microbial balance in the gastrointestinal tract [22; 23; 24]. To restore intestinal balance and support digestive health, it is recommended that consumers include probiotic milk products in their diet.