the.com/hobby horse

means A favorite topic or pet project that someone keeps returning to, often past the point of others' patience.

from Originally a small or spirited horse breed, the term shifted to the toy horse made of a stick with a horse's head — the kind children 'ride' by straddling and galloping around. That toy never actually goes anywhere; it just bobs in place while the rider keeps energetically pretending to advance. By the 1600s-1700s, writers (Laurence Sterne's 'Tristram Shandy' is the famous literary case) borrowed that image for a person's pet subject or obsession — something they mount again and again with great enthusiasm, while everyone watching can see they're not actually getting anywhere new.