Sealock Monster is a champ. I had the fortune of playing both at the Prerelease, and the Monster was either the best wall in my deck or the best creature in my deck, depending on who I was facing. A 5/5 for 5 is nothing to snort at, and green/blue can even give you an impromptu Island by
Nylea's Presence without needing to go monstrous.
The Hippocamp is pretty much a dud. At 2 toughness it just can't survive against anything it would be blocking. The surprise-untapping is also very lackluster. Attacking into 4 open mana in the format has been suicide since day 1, and it's far more rare that the untap clause will be as effective as a 5/5 beast. Agreed that if the Hippocamp had flying it'd be great though.