Cavalos-marinhos translates directly to "seahorses". This compound word is interesting because it literally means "sea horses", offering a visual and somewhat whimsical image.
In the context of the song, it appears as a sudden, almost out-of-place discovery at the very end, offering a moment of unexpected wonder and perhaps a glimmer of hope or distraction from the melancholic reflections on a lost love. It's a unique and memorable word that stands out from the more common vocabulary in the lyrics.
Legião Urbana's "Vento No Litoral" is a breakup song set on an empty afternoon beach. The narrator opens with a plan for the day — "De tarde eu quero descansar, chegar até a praia e ver" ["In the afternoon I want to rest, get to the beach and see"] — and lets the water do the emotional work for him: "Eu deixo a onda me acertar" ["I let the wave hit me"], "E o vento vai levando tudo embora" ["And the wind takes everything away"]. The coast isn't a backdrop here; it's the mechanism he's using to process a relationship that ended.
What he actually misses isn't the person's presence but the shared future they had planned together: "Dos nossos planos é que tenho mais saudade" ["It's our plans that I miss the most"], "Quando olhávamos juntos na mesma direção" ["When we looked together in the same direction"]. By the second half he talks himself toward acceptance — "Que a vida continua e se entregar é uma bobagem" ["That life goes on and giving up is foolishness"] — before the song loops back to the beach and the same wind carrying the same things away, seahorses and all.