Cuckoo, as expected, was the cuckoo bird that came out of clock in the Banana Splits Clubhouse. Whenever Fleegle would ask "What time is it, Cuckoo?" the entire front side of the clock would open up like a door and Cuckoo's head would spring out and give some joke or snarky reply to Fleegle's question.
Cuckoo resembles a multicolored feather duster with a long, thin beak, almost like a psycedelic version of Big Bird from Sesame Street.
Cuckoo seemed to be worked by mechanical methods, given the smootheness he sprang in and out of his clock. He could also spin his head wildy when he got excited. The 2012 Banana spilits revival, however used a new Cuckoo operated like a standard hand puppet, and retconned into a female.
Cuckoo, as well as Goofy Gopher, and the Banana Vac, were never used in the Gold Key Comics' published adaptation of the show , given that the stories took place with the Banana Splits on the road, rather than staying at the Clubhouse.
In the Banana Splits coloring books, Cuckoo was depicted as a full-bodied wooden mechanical bird, rather than the feathered version seen on the show. Given the coloring books were based on the Splits' original pre-production designs, This might have been Cuckoo's originally intended form as well.