Couple of points:
I did read a post about getting a dog as part of the list of defences in the home. Don't.
Not unless you actually want a dog in the first place.
Dogs are a big expensive, an added responsibility and getting one purely as a biological alarm system is not ethical in my view, unless you are going to give them the care and time they deserve.
If you are prepared to do so, then they are, of course, a great addition to any family...
Meanwhile, there are plenty of man-made alternatives if you are not, many of which have already been mentioned.
As far as I can tell no one has mentioned pepper-spray yet.
Surely, at least in the interim where training is not yet available, that is going to be effective against most people and can be held in in the hand very easily...
Just my 2 €c
When the right to effective self-defence is denied, that right to self-defence which remains is essentially symbolic.
Freedom: Please enjoy responsibly.
Karma. Another word for revolver: because what goes around, comes around!