Selection may favor greater investment into sexual ornaments when opportunities for future reproduction are limited (e.g., due to high adult mortality). However, a key driver of mortality, predation, typically selects against elaborate sexual ornaments. Here, we examine the evolution of sexual ornaments in a group of killifishes, which have marked contrasts in life-history strategy between species and inhabit environments that differ in their accessibility to aquatic predators. We first assessed if the size of sexual ornaments (unpaired fins) influenced swimming performance and found that larger fins negatively affected swimming performance. Second, we investigated whether the evolution of larger ornamental fins is driven primarily by the pace of life-history (i.e., investment into current vs future reproduction) or habitat type (as a proxy for predation risk). We found that males from species inhabiting ephemeral habitats with lower predation risk had both larger fins and more pronounced sexual dimorphism in fin size, compared to males from more accessible permanent habitats. Our results indicate that enlarged ornamental fins impair locomotion and evolve more frequently in environments that are less accessible to predators, but with no clear association to the pace of life-history. We provide a rare link between the evolution of sexual ornaments, decreases in locomotion, and natural selection on ornaments through vulnerability to predation.