Very little is known about the M-113 creature, except that it sucks salt from wherever it can. It is a shape changer. The original M-113 creature was part of a species that went extinct, because the species finished all the salt to be found on the planet M-113. It killed Nancy Carter, wife to a scientist who was working on a geological dig on M-113.
In passing, she was also one of Dr. Leonard McCoy's girlfriends. To suck salt, it would make a small hole on one's neck, then suck the salt. It was intelligent, because, having taken a crew member's shape, it found out that Nancy Carter had had an emotional connection to Dr. McCoy, so it took her shape, and tried to live off McCoy's bodily salt.