What is it?

From the official website of the Bocca della Verità

We think the Bocca della Verità is the God Oceanus (Οκεανοσ in Greek) and was probably the cover of a water collector inside a temple in the Foro Boario area. Oceanus, or Okeanos in the greek mithology, is the origin of all. Oceanus was a very special kind of river god, because the river Oceanus was the river that encircled the world, to which all other rivers flew. Achilles' shield had the mighty Oceanus all round the outermost rim. It was the supreme river god and for this reason we think the Mouth of Truth represents it, and was somewhere in the area of the foro boario, where the first Rome port was. We believe the Mouth of Truth is Oceanus because it doesn't have horns, but rather crab claws, as it is clearly visible in this equalized picture of the Bocca della Verita'. In the same picture are clearly evident: a fish in the left side and 2 round protuberances in the lower beard (T1,T2). We think those are the heads of 2 dolphins similarly to what we can see in a bronze head of Oceanus.
It could be that those are testicles and this could make sense according to Greek mithology. In the beginning there was Uranus, the sky, and Gea (or Gaia) the earth, his wife. Every night Uranus came to his wife and many sons (the Titans) and daughters were born, but Uranus kept hiding them underground preventing them from coming outside. Gea thought something should be done and asked her son Chronos for assistance. Chronos waited until Uranus came and cut his father's testicles and threw them into the Ocean. This could be a reason why we see these 2 testicles in the beard of Oceanus.
After looking well at the mouth of truth and having studied computer-enhanced images, we are inclined to think that these are dolphins.