What is the creature that grabs Luke by the leg and pulls him under the water in the garbage compactor on the detention level of the first Death Star?
All we see is one eye and one tentacle, which is not much to identify it by.
It was a Dianoga:
Dianoga were large cephalopods, capable of growing up to ten meters in length, though most specimens grew to approximately five or six meters. They had a single eyestalk protruding from their mollusk-like bodies, and seven suckered tentacles surrounding a fanged maw containing a sharp serrated probe.
And the reference of the creature attacking Luke on the Death Star trash compactor:
One dianoga was known to have boarded the first Death Star, and made its home in one of the space station's garbage compactors. When Luke Skywalker, Han Solo and Chewbacca were forced into the trash compactor during the rescue of Leia Organa, the dianoga attacked Skywalker. It pulled the young farmboy under the murky, polluted water, but was scared away when the compactor walls activated. The dianoga itself had managed to burrow into the wall, and so was relatively safe inside the compactor; that is, until the Death Star was destroyed.
Unfortunately, no one thought to plan for their escape, and Leia takes charge, blasting a hole in a nearby grate and jumping through while Han and Luke hold off a squad of stormtroopers. Chewbacca, Luke and Han all dive after the princess into the unknown.
Unfortunately, the grate covers a chute that leads to a garbage compactor that is also home to a resident dianoga.