Centaur: Str 18, Dex 14, Con 15, Int 8, Wis 13, Cha 11, +3 natural
Hippocampus: Str 16, Dex 15, Con 16, Int 5, Wis 13, Cha 8, +3 natural
Triton: Str 12, Dex 10, Con 12, Int 13, Wis 13, Cha 11, +6 natural
Average to Exceptional translates to Int 8-16.
AC = 15 with no armor suggested.
How about: Str 18, Dex 14, Con 16, Int 13, Wis 13, Cha 11, +4 natural (to go with +2 Dex and -1 size)?