hobbyhorse
1. [ noun ] a topic to which on constantly reverts
Examples:

: "don't get him started on his hobbyhorse"

Related terms: preoccupation
2. [ noun ] a child's plaything consisting of an imitation horsehead on one end of a stick; the child straddles it an pretends to ride
Synonyms: rocking_horse cockhorse stick_horse hobby
Related terms: plaything rocker