Poseidon Rex

Poseidon Rex is the name of both the titular monster of the sic-fi horror film "Poseidon Rex" as well as the species that it represents - a clear homage to Godzilla and dinosaurs such as the T-Rex from Jurassic Park.

History
Poseidon Rex was just one of many similar ancient creatures that lived in a mysterious pit of water so deep it was considered "bottomless" - this, of course, was not true and at the bottom of said pit many eggs were found and thus Poseidon Rex was revealed to have been living their for centuries.

When humans finally encroached on its watery domain the predatory creature saw an opportunity for new prey and rose from the waters to attack, devouring anything it came across and spreading a path of death in its wake.

Eventually the adult Poseidon Rex is killed via a rocket-launcher but deep below an egg hatches and a juvenile Poseidon Rex is seen, swimming its way towards the surface - presumably to begin another rampage and showing that the species now considered humans as part of their prey.

Abilities

 * amphibious (Poseidon Rex and its offspring are all capable of existing both in the depths of the ocean or on dry land, with no ill-effects, in fact despite being an ocean-dweller the creatures have powerful legs and feet, showing they are also evolved for walking on ground as much as swimming)
 * superhuman strength (Poseidon Rex is immensely strong due to its size and muscular build, even baby versions of the creature can overpower full-grown humans with ease.)
 * superhuman durability (they are immensely durable with only powerful military-grade weaponry such as rocket-launchers being sufficent to kill them, even a baby was able to withstand multiple gunshots without even flinching.)
 * superhuman size (while they start off rather small Poseidon Rex grow to extremely large sizes, easily dwarving humans when in adult stage - they can swallow humans whole and completely sink medium-sized boats.)

Trivia

 * even by the realms of B-Movies the general story and effects of Poseidon Rex are widely panned as ridiculous - though often this is deliberate, especially with the rise of mockbusters and "so bad it's good" subgenres of B-Movies.
 * Poseidon Rex is meant to translate as "Ocean King" or "King of Ocean".
 * the movie often depicts the monster walking on water, despite giving absolutely no logic as to how or why it is able to perform such a feat.