Kyogre is the god of the sea, a fearsome beast embodied in waves which devastate cities whenever it awakes. It lies dead but dreaming beneath the waters of Sootopolis, and will rise again when the stars are right. Gyarados are fearsome in appearance, and indeed in battle, but they live in the deep oceans and seldom venture towards the surface, although when they do it of course is terrifying. But it is Tentacruel, not Kyogre or Gyarados or even Poliwrath, which sailors fear the most.
Part of this is a simple matter of biology: a Gyarados must pierce a ship to sink it, but a Tentacruel can simply use its eighty tentacles to drown an entire ship's crew. Their venom eats through wooden ships with astonishing speed, and their small center and quick movements make them a difficult target to hit. Rare is the ship which survives an attack from a mature one even today, and in the old days it was even rarer.
But most of it is that they will do it: Poliwrath are rarely dangerous outside of human hands, but Tentacruel nurse an ancient grudge from their "defeat" in the war of land and sea (in reality it was a stalemate, even a slight victory, but wars can easily be mythologized) and view every sailor as an invading force on a victory lap. And there is also the fact that, of all the water pokemon, it is only Tentacruel and those owned by Team Aqua who think the war should not have ended, and wish to start a round two. Most of those in the sea correctly realize that man has sided with Groudon, and that man rules the world now, but every now and then, giant Tentacruel sack a coastal village and the sea weeps for fear of retaliation.
I have written two entries for this pokemon. Below is the original, which was fairly unpopular, because it dealt with Tentacruel bestiality.
The standard anti-bestiality laws contain an exception for women (but not men) who have sex with Tentacruel. Most people believe the laws were put there due to concerns about rape: Tentacruel are a large pokemon who often have their way with women, including oftentimes their female trainers. However, Tentacruel are not the only pokemon to engage in this behavior – Houndoom and Mightyena do it commonly, and most large pokemon with proper appendages can be taught to do so by a sufficiently sadistic trainer, and will continue even after being freed.
This is a common misconception. Language elsewhere in the statute makes it clear that only willing intercourse with pokemon is forbidden, so as not to punish the innocent. The roots of this exemption lie far deeper in our culture, from religious rituals in ancient times when virgins (or at least their virginity) were offered as sacrifices when the rare giant Tentacruel menaced our cities, to ages where Tentacruel sex became an almost accepted (if never discussed) way to meet women's needs without risking pregnancy in a polygamous society, and was punished far less severely than trysts with other humans. And in modern times, the pornography industry thrives on it; indeed, censorship legislation has had the paradoxical effect of making this easier to produce then human-on-human porn.
Modern, "enlightened" activists wish to ban this, either on decency or animal rights grounds, but they have made no progress. Like incest between cousins, Tentacruel sex is one of those things many may find disgusting, but has roots far too deep to ever truly go away.