Is Ika Musume the only humanoid squid creature? In Shinryaku! Ika Musume (Squid Girl), the protagonist is a strange creature called Ika Musume who looks almost like a young girl, but has a number of strange squid-related abilities, such as spitting ink and using her hair as tentacles. She is also intent on invading humanity.

Are there any similar squid-like humanoids in the series, or is she the only one of her kind?
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.