Long-term multi-animal tracking in densely group-housed agricultural settings is critical for automated behavior monitoring and early anomaly detection in precision livestock farming. However, it poses significant challenges due to persistent occlusions from feeders and water dispensers, high inter-individual appearance similarity, and drastic visual changes across day and night cycles. Existing multi-object tracking datasets rarely capture the combined difficulty of these real-world conditions. To address this, we introduce OinkTrack, a large-scale benchmark for continuous multi-pig tracking in commercial farm environments. The dataset comprises over five hours of annotated video across sixteen sequences, covering day, night, night-to-day, and day-to-night transitions. Each sequence ranges from one minute to one hour, featuring an average of thirty-six pigs per frame. In total, OinkTrack provides 573,700 bounding boxes linked to 574 consistent pig identities. It enables detailed behavior analysis under varying lighting and crowding conditions. We describe the data collection and annotation process, present statistical insights into tracking difficulty, and benchmark 11 state-of-the-art tracking methods. OinkTrack provides a robust foundation for developing long-term tracking models and supports downstream applications such as individual activity profiling and early detection of abnormal behavior in real-world, high-density animal populations. The complete dataset and supplementary materials are publicly accessible at https://leohuang0511.github.io/oinktrack-page.