Which preposition to use with watchdogs

of Occurrences 2%

I added: "Bartholomew Storrs, as sexton, has constituted himself watchdog of our graves and censor of our dead.

in Occurrences 1%

As a half-grown boy I killed our good, kind watchdog in one of my fits of rage for some trifling offense, and I have never ceased to regret it.

than Occurrences 1%

In determining the choice of a breed it is to be remembered that some are better watchdogs than others, some more docile, some safer with children.

Which preposition to use with  watchdogs