The trebius is found in some medieval encyclopedias. It is a black fish that is a foot in length and five inches wide. It changes its color to white in the winter and black in the summer. A part of it preserved in salt can draw up gold that has fallen int a well. It is the only fish that lays eggs in a nest. It pierces ships with its sharp beak.

The marvelous trebius is a fish that does not actually exist; Thomas of Cantimpré "created" it through a mistaken reading of two passages from Pliny the Elder (see Sources). He mistook Pliny's reference to something a Roman, Trebius Niger, said about the murex, the sea-snail, and assumed the name of the fish was trebius. From that passage he deduced that the trebius was a foot long and five inches wide; that a part of it preserved in salt could draw up gold that had fallen in a well; and that the trebius attacks (actually "hinders") ships. He also read the next passage about an unknown fish called maena as part of the description of the trebius, which added the information that the trebius changes color. He then included Pliny's description of the lamprey, saying it is the only fish that lays its eggs in a nest.