**Background:** The mental experiences of animals define their welfare status. The Five Domains Model provides a holistic framework linking physical/functional states (Domains 1–4: Nutrition, Physical Environment, Health, Behavioural Interactions) to inferred mental experiences (Domain 5). For free-roaming wild horses, practical indicators have been described, but their scientific validation—linking measurable indicators to physical impacts and then to mental experiences—has not been systematically compiled. This review aims to validate previously described welfare indicators for free-roaming horses and distinguish between 'welfare status' (directly reflecting mental state) and 'welfare alerting' (indicating risk) indicators.
**Methods:** The authors reviewed published literature on free-roaming horses and related species, applying a two-step validation process adapted from Beausoleil and Mellor (2017). Step 1 validates the link between observed indicators and physical/functional impacts using pathophysiology, mechanistic understanding, elimination studies, and coherence across modalities. Step 2 validates the link between physical/functional impacts and mental experiences using affective neuroscience, human comparisons, and intervention studies. Indicators were categorized as welfare status (animal-based), quasi-welfare status (resource-based with strong evidence linking to mental experience), or welfare alerting (indicating risk). The review covers negative and positive mental experiences across all four physical domains.
**Key Results:** The review validates indicators for a wide range of negative mental experiences: thirst (water-seeking behaviour, water availability), hunger (low body condition score, food availability), cold discomfort (shivering, ambient temperature below thermoneutral zone, wet coat, poor body condition), heat discomfort (sweating, high ambient temperature, humidity, insufficient shade), localised pain (wounds, quidding/food pouching for mouth pain, blepharospasm for ocular pain, lameness for limb pain), non-specific chronic pain/malaise/fatigue/exhaustion (head-lower-than-withers posture, ears back, facial grimace, reduced alertness/dullness/apathy, very thin/emaciated body condition, muscular weakness), breathlessness (increased respiratory rate and/or effort), social pain (isolation from conspecifics), anxiety, and fear (threatening environmental events). Positive mental experiences validated include pleasures of drinking, mastication, post-prandial satiety (standing rest after eating), cooling and warming (immersion in cool water, radiant heat), vitality of fitness (active locomotion in healthy horses), exercising agency (exploration, selective foraging), and affiliative social interactions (reciprocal grooming, maternal care, play). The review introduces the term 'quasi-welfare status indicator' for resource-based measures like water availability and ambient temperature, where strong evidence links them to mental experiences. Welfare alerting indicators (e.g., low ambient temperature, high faecal egg count, small band size) signal risk but do not directly reflect current mental state.
**Clinical Implications:** This validated framework enables transparent, scientifically justified welfare assessment and grading in free-roaming horses using the Five Domains Model. It provides a practical tool for conservation managers, researchers, and policymakers to evaluate and improve welfare in wild horse populations. The distinction between welfare status and welfare alerting indicators improves the precision of inferences about mental experiences. The approach can be adapted for other free-roaming species, promoting evidence-based welfare assessment in wildlife contexts. The review also highlights the value of studying free-roaming animals in optimal conditions to understand positive welfare states.