large mythical fish, proper name
鯀 refers to a large mythical fish, and is also used as a proper name. The two senses are connected by the legendary figure Gun (鯀) in Chinese mythology, who is associated with a giant fish or turtle.
鯀 combines 魚 (fish) with 系 (lineage, system), likely as a phono-semantic compound where 系 hints at the sound and 魚 indicates the meaning. The character is historically tied to the name of the mythological figure Gun.
A fish (魚) connected to a lineage (系) suggests a legendary creature passed down through myth: a giant mythical fish.
For コン, imagine a con artist trying to sell a giant mythical fish: con -> コン, and the fish is too large to be real.