The day-fish (dies piscis) is found in some medieval encyclopedias. It only lives for one day after it is born. Thomas of Cantimpré says it has two feet and two wings and has no blood; Aristotle says it has wings and four feet.

The day-fish may be the mayfly (modern taxonomic order Ephemeroptera), which in adult form lives for a very short time. Aristotle's description suggests that an insect is intended rather than a fish.