Comparison between the photometric redshifts (z_p) and spectroscopic redshifts (z_s) for the simulated Horizon-AGN galaxy sample. Each panel shows a two-dimensional histogram with logarithmic colour scaling and is annotated with both the 1:1 equivalence line (red) and the |z_p − z_s| = 0.15 (1 + z_s) outlier thresholds (blue) for reference. Photometric redshifts are computed using both DES/Euclid (left) and LSST/Euclid (right) simulated photometry, assuming a Euclid-based magnitude-limited sample with VIS < 24.5.