The demand for novel sources of marine oils, which contain polyunsaturated fatty acids (PUFAs), has increased due to the realization of the importance of PUFAs, e.g., docosahexaenoic acid (DHA), in the human diet. However, the natural supply is limited. By-product peptones (BYPP) intended as a growth medium for the PUFA-producing strain Sicyoidochytrium minutum of family Thraustochytriaceae were produced after several experiments on the pancreatic digestion of bovine lungs and spleens. S. minutum was able to grow in a medium containing BYPP made from the pancreatic digestion of lung and spleen with glycerol, resulting in 1.14 ± 0.03 g cell dry weight (CDW)/L and 1.44 ± 0.24 g CDW/L, respectively, after 5 days of incubation at 25 °C, compared to 1.92 ± 0.25 g CDW/L in Basal Medium (BM) containing tryptone, peptone, and glycerol. The lipid content, obtained after growth in lung BYPP media with glycerol as a carbon source, was significantly higher (28.17% ± 1.33 of dry weight) than in the control basal medium (BM) (21.72% ± 2.45); however, DHA as a percentage of total fatty acids was lower in BYPP than in the control BM (25.24% ± 1.56 and 33.02% ± 2.37, respectively). It is concluded that low-value by-products from abattoirs can be used as ingredients for the cultivation of oligogenic Thraustochytriaceae.