Sea horses? This is also Day’s suggestion: “The central crowned figure rising from the waves can be claimed as Neptune in spite of the absence of a trident. Just above are his two sea-horses , and the symbols depicted on their heads are interesting. That on the left has a dove bearing. an olive branch, while that on the right has a St. Andrew cross.”
Cooper -rather- sees in these two figures God the Father and the Holy Ghost and Day’s “Neptune” is the resurrected Christ. In combination this is a depiction of the Trinity.