The Hydra, an ancient serpent-like chthonic water beast that possessed many heads , the poets mention more heads than the vase-painters could paint, and for each head cut off it grew two more, it's poisonous breath and blood were so potent, even its tracks were deadly. The Hydra of Lerna was killed by Hercules as the second of his Twelve Tasks
The Hippocampus is a glamourous animal of the ocean. They were the pets of the sea and horse god, Poseidon and would draw his chariot across the sea. They have the upper-half of a horse and the lower-half of a fish or dolphin. They love being out at sea and are fairly strong creatures with lots of stamina. They are devotional to their god and like to spend time with their fellow Hippocampi.
Tree nymph - DryadA dryad (Δρυάδες, sing.: Δρυάς) is a tree nymph, that is a female spirit of a tree, in Greek mythology. In Greek drys signifies "oak". Thus dryads are specifically the nymphs of oak trees, though the term has come to be used for all tree nymphs in general. They were normally considered to be very shy creatures, except around the goddess Artemis, who was known to be a friend to most nymphs.