"...Curious Squid, so called because, as well as being squid, they were curious. That is to say, their curiosity was the curious thing about them." (Jingo)
Curious squid are squid in the Circle Sea, notable for terrible foul taste and for their interest in anything new that presents itself in their environment. Curious squid are fished for connoisseurs who want to taste a course that contains no trace of the squid. Curious squid live near the underwater landmass that, when it occasionally floats up to sea surface, is known as Leshp, which was an object of dispute for land-dwelling humans in Jingo.
One reason that they are so easy to catch is that they often become very briefly curious about the shiny pointy hook that comes towards them. This shows that while curiosity may be the handmaiden of intelligence it doesn't work very well as its substitute.
Two fishers of curious squid that we learn a lot about during Jingo are Solid Jackson and Greasy Arif, who both feel that they are the first person to set foot upon Leshp when it reappears, claim it for their respective nations (Ankh-Morpork and Klatch), and set in motion the chain of events chronicled in said book.