Saddle Cat

So, it's clearly a part of the zeitgeist.  Or at least was.  It probably still should be, what with YouTube being one giant shrine to Bastet.  So yes, it should be okay to have your character to tie a feathered mousy to the end of his lance and ride into battle upon a couch leaping, yarn chasing, cheezburger hazzin' giant kitty.

The Good:

•  Talk about riding a jumper.  Saddle cats are capable of leaping twice as high and twice as far as a light warhorse and can charge their regular move distance straight up a masonry wall or wooden palisade.
• Causes abject terror in ratmen
• No need to dismount to be sneaky

The Bad:

• They can only be ridden 2 hours a day - they choose which 2
• The fish bill.  Oh dear merciful Bastet, the fish bill.  Seems they'll go through a barrel of salted fish every couple of days, plus hard tack, jerky, and whatever the local farmers would prefer they not catch.
• Prone to dragging grooms around in their mouth without provocation.

The Ugly:

• Lock up the Familiars
• Occasionally fall through ceilings
• Dragging the groom around means it's *your* turn to clean the enormous litter box

In game terms, I'd just give them the same stats as a cavalry horse, but with twice the movement capability (jump or charge), twice the damage, no endurance, and enormous upkeep costs.  They should also be capable of stealth and very hard to surprise.