The "mirror-scaled" oracular koi in the pond at Tom and Carl's house. Doitsu is male.
The koi's name is derived from the type-name of the original mirror-scale carp, a central European mutation imported into Japan from Germany in the early 1900's ("doitsu" being a Japanese form of deutsche). (AWOM)
(See also Akagane: Koans: Showa.)