Deep Ones are one of the more famous and iconic monsters to come out of the Cthulhu Mythos. First appearing the Lovecraft story "The Shadow over Innsmouth", the Deep Ones are a race of amphibious humanoids - described as looking like a humanoid amalgamation of fish and frog - who inhabit the depths of the oceans, where they worship Cthulhu and other Great Old Ones. Ironically, Deep Ones are amongst the more human denizens of the hidden darkness; they pursue a lifestyle not that different to humans, seeking comfort, wealth and the chance to do what their cults consider "good deeds". As such, they are capable of conducting diplomacy with humans and forming alliances; the precedent set by Innsmouth amounts to the Deep Ones offering good harvests of fish and bounties of treasure retrieved from the ocean's bottom in exchange for conversion to their faith and other acts of allegiance.
One of the more unusual aspects of Deep Ones is that they can interbreed with humans. These Deep One Hybrids spend the first fifty years of their life as humans, albeit with a pale and/or greenish tinge to to the skin and somewhat batrachian features; the so-called "Innsmouth Look". As they approach their 50th birthday, the Hybrid's features become more and more inhuman, until they reach the age of fifty and transform into a full-fledged Deep One in turn. As Deep Ones possess immensely long, if not infinite, lifespans, this is seen as a great blessing; once they have transformed, the former hybrid is functionally immortal.
So why are Deep Ones seen as the most evil of the Mythosian monsters? Let's just say that Lovecraft had some...peculiarities (as in his sole criticism of Adolf Hitler was how optimistic the guy was), and people still tend to buy the unflattering light he portrayed them in.
Monster Hunter International Deep Ones are far more monstrous. They raid ships on the surface and shoreline to forcibly implant their eggs in human hosts that burst open Aliens style to spawn horrifying monsters. They are so widely despised every nation on Earth with a submarine has agreed to cooperate in dropping depth charges on their cities when they are discovered.