Nowadays, the co-processing of middle distillates with vegetable oils is well established at an industrial scale. However, it is necessary to utilize waste oils (used cooking oils/animal fats) to obtain second-generation biofuels. The present work studies the suitability of animal fat as a feedstock for co-processing with atmospheric gas oil, paying particular attention to catalyst activity in terms of hydrotreating effectiveness and hydrotreated gas oil quality, including cold flow properties. The co-processing experiment was carried out in a fixed bed reactor for more than 260 hours, under industrial operating conditions (362–377 °C, 5.8 MPa, LHSV 1 h⁻¹, H₂ 317 Nm³ m⁻³) and an atmospheric gas oil/animal fat ratio of 90/10 wt%. The addition of animal fats did affect the hydrodesulfurisation efficiency (>99.6%). However, the co-processing affected the hydrodenitrogenation efficiency (>94.0%) more and the hydrotreated gas oil quality in the longer term. Furthermore, in the case of cold flow properties, 100 ppm seems to be the optimal value for depressant additivation to improve significantly the CFPP value (up to 10 °C). Overall, our results suggest that, while it is necessary to implement a previous cleaning step, animal fats are a suitable and promising blending feedstock candidate for co-processing with medium distillates to produce advanced biofuels.