monomaniac (n.)

"insanity in regard to a single subject or class of subjects; mental action perverted to a specific delusion or an impulse to do a particular thing," 1820, probably on model of earlier French monomanie (1819), from Modern Latin monomania, from Greek monos "single, alone" (from PIE root *men- (4) "small, isolated") + mania (see mania).

Men of one idea, like a hen with one chicken, and that a duckling. [Thoreau, "Walden"]