Short answer

Hippocampus is the seahorse emblem Omega stamps on Seamaster casebacks. "Fake hippocampus 2013" is a search phrase used for Seamaster watches sold as 2013 production whose seahorse stamp does not match Omega tooling. Omega publishes no document with that name. The phrase comes from auction listings and forum posts. The 2013 label points at a production range, not one model.

The emblem and where it sits

Omega put the seahorse on the Seamaster 300 caseback in 1957. The mark moved with the line after that. On a closed caseback the seahorse sits inside a circular border with the water resistance text below it. On a sapphire display back the seahorse appears on the oscillating weight. Crown and clasp carry a smaller version of the same mark.

Checks for a 2013 caseback

  1. Strike depth. Genuine stamps are cut into the metal at even depth. Counterfeit marks read flat and shallow.
  2. Fins and tail. The dorsal fin shows separate rays. Replica dies blur the fin into one shape and round the tail tip.
  3. Border ring. The circle around the seahorse stays concentric. Off-center rings show on 1:1 copies.
  4. Text baseline. "Seamaster" and the depth rating sit on one baseline and use the same font weight.
  5. Reference number. Omega codes use six groups and 14 digits, for example 231.10.42.21.01.001. A five-group code or a missing group points to a fake case.
  6. Serial number. Genuine cases carry a serial on the lug or caseback rim. Copies repeat one serial across many units.

What changed in 2013

Omega introduced the Master Co-Axial movement family that year. Listings that pair a 2013 caseback with an older movement reference are mismatched. Catalogs list the reference number, the case size, and the movement. The date in a listing title does not prove the caseback date.

Limits

No public archive lists every seahorse die Omega used. A stamp that differs from a reference photo is not proof of a fake. Service casebacks and replacement parts create legitimate variation. For one watch, compare the stamp with a known genuine example of the same reference and year. Omega does not publish die drawings.