What's the big deal about guard dogs? 
Why doesn't anyone train horses to 
(ahem) take care of tresspassers?
 

People look at horses and say, oooh, aren't they pretty! However, anyone
who has ever gotten a hoof sandwich has, shall we say, a larger opinion. 
For starters, the way horses just play could kill a human. Obelix, on the
left above, weights approximately a metric ton. When he play fights, it 
sounds like thunder. The reason horses rarely get hurt when they play is
their hides are, well, tough as horsehide. 

On the left above, Viking, a Paso Fino stallion weighing 800 lbs, and Danny,
a Percheron weighing 1500 lbs, are playing. Just playing.  Bottom
right they end their play with a  side-by-side gallop.
Video of Viking and Danny play fighting. 
More play fighting. 
Playing kissy face.

Danny wants to see whether Flair (an elderly, half blind gelding) or Viking
have yummier food than his. Viking and Flair both remind Danny that he is 
low horse on the totem pole. 

Viking punishing Danny for rude behavior. 

More violence as Viking insists that he is the boss. 

On the fourth day that Danny shared a pasture with Viking and Flair, 
Danny went beyond rude. The following videos show hostile behavior 
as Danny tries to batter his pasture mates into submission.

Danny's ears are pinned back. Flair grunts with rage as he hits 
Danny's chest with both his hind hooves. After Danny and Flair back
off from each other, Viking moves around uneasily as he plots how to 
clobber Danny big time.

A few moments later, Viking attacks and Danny fights back. 

Minutes later I removed Danny from this pasture because I feared that one of the horses was about to get seriously hurt.

This is just a hint of why, for thousands of years, far more horses than dogs have been used in war. Giant horses like Danny the Percheron and Obelix the Belgian carried knights in armor. Small, agile horses like Viking carried Berber nomad warriors and the Spanish Conquistadores.

Today, police horses are trained to trample rioters under their steel-shod hooves.

By the way, I no longer own Danny. Today he is in training to become a police horse. Watch out, rioters!

Still want to commit crime? Better get a lawyer... and plenty of health insurance.