My favorite headcanon is that McGonagall actually had a pet cat that looks EXACTLY like her animagus from and lets it wander around Hogwarts to our all her coworkers on the edge.
It's only after it uses in Snape's office, jumps up on Flitwick's Lao and starts kneading, and serenity and throughly licks it ass in front of Sprout they begin to suspect. Dumbledore of course knew the whole time and played along having entire one sided conversations with it, letting it sit on meetings McGonagall didn't particularly feel like attending, and occasionally charming objects like quilts and parchment to make it seem like the cat was grading.
