Back ground - ridden for 20 years been involved with breaking, training analysing and watched a lot of horse antics.

Horses are flight creatures - they gallop to get away from things. Man trains that to other uses. A horse will run through things - if its scared enough and with many other horses it will run through fences, hedges, ditches and such only stooping when it is too injured to continue. Or too knackered

Myths:

Horses don't like galloping down hill - try it - they don't. IMHO any cavalry charging downhill should be immediately disordered. The zig-zag downhill.

Truths

Horses like galloping uphill -like hares - an example of cavalry charging up a hill - charge of the HEAVY brigade at balaclava (the light brigade were plain dumb)

Ever wondered what blinkers were for....where the ones race horses have came from.....the ones on armoured war horses had blinds so the horse couldn't see where it was going. These were closed just before charging - horsemen at this time used 'sharp' spurs (and I mean points that made the horses bleed) to cause pain to make the horses charge no what

Five men stopping a war horse at full charge - no chance

Five men in full armour - weighs 1 tonne and is stationary
One fully armoured horse and rider - 1.2-5 tonnes

Einstien e=mc2.

Horse
c for a horse at gallop = 60kph(40mph)=15metres/sec
e=1200x15x15=shitloads

Men
c=0
e=not a lot

Five men have NIL chance of stopping it. They may kill it, injure it, cause it to fall, the horse and rider may be thrown to the ground but the next horse is through.......that's what is supposed to happen next...the five men are dead or at best still flying through the air backwards.

Marshall Ney at Waterloo got through 5 horses (mad bastard :) it didn't stop him.

Myth (unrelated but sorry was mentioned above)
No square at Waterloo was broken - many were....just not enough. The French cavalry stopped because the captured their objective - Wellington's guns - but they didn't spike them...why is another story because if they had Napoleon may well have won. The English squares were behind the artillery and the French cavalry engaged them piecemeal. They were also fighting English cavalry as well and the English were also good at defending yadda uyadda yadda........ They didn't succeed because they charged unsupported...another long story....

Anyway - don't underestimate a charging horse - if you have ever stood in the way of one - you get out of the way fast :)...and often need clean underwear afterwards :)