"pet subject" is a correct and usable phrase in written English.
It is typically used to refer to a topic or issue that someone is particularly passionate or knowledgeable about. Example: My grandmother's pet subject is gardening; she can talk about it for hours and has a beautiful garden herself.
It has always been his pet subject (and the one where he speaks most fluently).
Despite scoring a few points against Sanders, Clinton appeared weakest on her own pet subject in Sunday's debate.
In 2011, Greenfield was interviewed for New Scientist about what by now had become her pet subject.
Another pet subject is the need to improve British standards in maths for fear that children are falling behind those in Asian countries.
Republicans, too, shied from Ryan's politically toxic pet subject, even while positioning him to amplify his arguments (he'll be chairman of the Budget Committee).
VERY CASUAL (Koyama, paper, $15) collects perverse, funny, haunting stories by the Canadian cartoonist Michael DeForge, whose pet subject is the overwhelming creepiness of bodies.
In a positron emission tomography (PET) study, subject-selected music, which reliably elicited subjective reports of chills, was compared to music that elicited chills in others (Blood and Zatorre, 2001).
Serious scholars do not always appreciate Kaplan's take on their pet subjects, and sometimes they have a point.
In the world of classical music and opera, these are pet subjects for tournaments of aesthetic one-upmanship.
It is a mistake to lecture people on your own pet subjects, as this columnist has discovered.
I also kid myself that reading about gender in science or science communication (my pet subjects) is a hobby, and therefore acceptable to do outside my logged hours.
