I'm not sure if this creature has been suggested, but the following would be nice to tame.

Hippocampus - From the Latin or Greek word for "seahorse", this aquatic creature would be inspired by the Hippocampus found as statues or heraldry. The only difference is that these beasts would be capable of swimming or diving.

Capricorn or Capricornus - Call it a goat "cousin" of the Hippocampus, but smaller and with the upper half of a goat instead of a horse.

New Feature - Swim or dive as actions if you own a beast capable of swimming especially when exploring a randomly generated island. There could be enemy creatures to battle like giant squids, kraken, or serpentine sea dragons. This I feel because Beastkeeper is long overdue for new tameable beasts and features.

The game is fine as is if none of the suggestions are possible, however, it would be more engaging if there was more to do under the exploration tab, as well as making new higher-leveled blueprints and schemes to craft once some of use have reached the maximum level of the riding hall, town hall, stable, and warehouse.

Another idea is giving shells more uses besides as an ingredient for crafting special tack. Maybe the shells could be reduced to a powder to be used for specialty potions?