Q:

Why was Kate the only one subjugated by the Zygons?

In The Day of the Doctor, the Zygon go around stealing identities.
When a Zygon took Kate Steward's identity, she was subjugated:

Osgood mentions that the Zygon need to keep their victim subjugated to refresh the copied image.
Well then, why weren't the other victims also subjugated? Like Elizabeth and Osgood. Maybe because they managed to escape - but in that case, why were their Zygon counterparts capable of maintaining their images, if their victims were not subjugated?
And how come that after freeing Kate, the Zygon that copied her was still perfectly fine? Maybe there is a time limit - is that mentioned?
... and the Doctor's horse!

A:

They just need the source to be alive. They needn't be necessarily "subjugated". That's just to be safe so that both don't come up against each other by mistake.

