Ramulkens are a species native to SR388. Their bodies are protected by a hard shell, and beams will not affect them. Their only weak point is their legs. Samus encounters the first one at the summit of a tall Chozo tower. In Metroid: Samus Returns, Ramulkens are purple in color and appear in Area 3. They roll into a ball and quickly charge at Samus, which she can deflect using her Melee Counter or jump over.
Metroid II manual
"Their bodies are protected by a hard shell, and beams will not affect them. Their only weak point is their legs!"
"Some of the most aggressive creatures on SR388, Ramulken look a bit like hermit crabs, but they don’t act like them. They skitter about an area until approached by a threat, at which point they duck into their shells and begin spinning furiously at their target. The meaner sub-species is even worse. Your best bet is to buckle down and aim for that Melee Counter, or run and escape the fight entirely. All other options lead to a long, dangerous fight, especially if multiple enemies are involved."
Walkthrough (p. 95)
"These shelled creatures are what you’d get if you mixed a Halzyn with a Gravitt. Ramulkens scuttle around peacefully until approached by a creature they perceive as a threat—Samus, in this case. When that happens, they quickly enclose themselves in their shells and begin spinning rapidly, then let loose in the direction of their enemy. They move pretty fast, so watch out. Listen for that telltale click of a counterable attack."
Walkthrough (p. 142)
"Stronger Ramulkens are hard to defeat and can spin at you three times consecutively. Your best bet when dealing with them is to stand your ground and Melee Counter them. Don’t try to shoot them with missiles while they spin; this is a good way to take damage, or to drag out the fight much longer than necessary."