guard dog
US /ˈɡɑːrd dɔːɡ/
UK /ˈɡɑːrd dɔːɡ/

1.
a dog trained to guard property or people
:
•
The factory employed a large guard dog to deter intruders.
•
Our new puppy is too friendly to be a good guard dog.
a dog trained to guard property or people