Is Ika Musume the only humanoid squid creature?
As far as I know, she's the only squid-like humanoid shown in the series. Though there is a miniatured version of her that appeared on people's dream, mostly Sanae's (source). In the last episode of Squid Girl, she met a girl named Kozue Tanabe which is implied that she also comes from the sea, as she has a hat similar to Squid Girl's and makes statements that indicate she isn't human. The first syllables of her name spell out 'tako' (たこ?), meaning octopus.