Most likely because, just like his big brother, he's a Kaiba.
You see, it's actually a pun. "Kaiba" uses characters for "sea" (Kai) and "horse" (Ba), although "Kaiba" doesn't mean "seahorse". (Japanese language is that complex lol)
Anyway, that's why Seto Kaiba has Tatsu-no-otoshigo and Kaiser Sea Horse.
Anoymous
