Squid or octopus.
You shouldn't call them anything.
Every living creature evolves to become more suited to their environment and ways of living. The deer have very strong and powerful legs for running away from predator's.
A steer with two short legs and two long legs is "lean beef."
A crane fly/daddy long legs put there teeth cannot penetrate through skin.
The small bird with long skinny legs known for its graceful movements and distinctive call is the sandhill crane.
A creature with 100 feet would be classified as a centipede. Centipedes are arthropods belonging to the class Chilopoda, characterized by their long, segmented bodies with one pair of legs per segment. Despite their name suggesting they have 100 legs, centipedes can actually have anywhere from 30 to 354 legs, depending on the species.
sticky browny long legs weirdo
It's an animal of course, but a Jerboa is in the rodent family. A Jerboa has small ears like a mouse and a long tail and long legs, which lives in Arabia. It is a unique creature.
The creature you are describing is likely a type of arachnid called a whip scorpion. Whip scorpions have a body similar to a spider, with long, thin legs and antenna-like structures on their heads.
The Yeti Crab is an odd looking creature that has fuzzy yellow hairs that cover its legs. The Yeti Crab averages about 5 inches in length.
A creature of the water's surface is the water strider. It uses the surface tension of the water and its long legs to actually walk on the surface.
There is nothing "conventional" about this creature, even for mythological beasts. It does not show up on any search of mythology or cryptid data bases. If you invented it, it is up to you to name it.