it looks like a horse, but the tail on the left side of the image shows it is actually a hippocampus (aka hippocamp aka sea-horse aka merhorse). Like a mermaid, but with a horse front-end.
This imaginary beast is shared by Phoenician and Greek mythology. Casually lounging on this particular hippocamp is Thetis, a nymph and the mother of Achilles who recently lost his armour. Mom is busy delivering him a special new shield: The Shield of Achilles, described in Homer's Iliad.
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