This creature (Jurassic Artemia) was not born in the valley, but nearby in a resort created by Melvin and it appeared to have a grudge against Melvin. The creature was created because Melvin borrowed (stole) water from a lake in the valley, unaware the water had been contaminated with Mixmaster.
It should be noted in this case Mixmaster didn't create a hybrid in the regular sense, but rather it developed a local shrimp into its prehistoric form. This allowed the creature to survive when Melvin attempted to cleanse the lake of anything he thought would "pollute" the lake and had the unforeseen side effect of killing almost everything in the lake robbing the shrimp of its food and seriously ticking it off.
It crawled at the way down to the river, where Rosalita showed up with her cattle fence and trapped it in. Twitchell drove his car into the other side of the river, then Tyler, Rosalita, the sheriff and a deputy began to throw dry ice into the river, freezing it to death.
The Original Giant Shrimp
While producing Tremors II, Stampede and Universal also developed Tremors as a TV series. The series premise was that, after their success in fighting the Graboids, Val and Earl would be sought out by the readers of tabloid newspapers, believers in UFO’s, etc. to battle other little-known creatures.
Val and Earl are called to battle it when it suddenly begins attacking boaters in a recently flooded dry-lake bed. The Designers were borrowing on the desert brine shrimp, which lay dormant for years until brought to "life" by the infrequent rain.
The original design seen in the sketch is a variant on the prehistoric shrimp Anomalocaris, which would be "giant" in the show because the animals it is based upon did not exceed 1 meter in length.
It was a normal shrimp that was mutated form the Mixmaster to enhance its size to be a prehistoric form.
Aquatic: Through being a shrimp creature, it will breath underwater for a certain period of time.
Shell Armor: It is shown to be quite bulletproof from the Sheriff's Gun as he been shooting it multiple times, it didn't seem to faze by the bullet (although it was rubber bullets the Sheriff use and in water for the most part)
Strength and Combat: It seemed to have enough strength to hold down two people at the same time with its pincers but trying to pull down third like Tyler who was fighting for his life proven to be unable to hold as it was stated to be quite powerful enough to rip Tyler's arm off.
Intelligence: It is proven to stay out of clear from the shore such as a crocodile waiting for its prey to get nearly to the water to pull down its prey easily.
Compund Creation: If it’s body dies as animals that eat will contain the mixmaster to testily future monsters. Though even it’s blood could contains the dna to spread