I'd go for +50% Hit Dice rather than a straight +3 HD.
e.g.
3 HD Leopard => 4 HD.
5 HD Lion => 7 HD.
That suggests a "Sabre Toothed Jaguar" would have 6 HD, since they have 4 HD in AD&D.
Fair enough, but let's round up on the +50%. Just for the heck of it.![]()
So the stats I whipped up above would actually be the "sabre toothed jaguar", and then we downsize it to 4 HD for the "saber toothed leopard"?
Yes, let's increase the bite, though one step for the lion is 2d6.
Definitely Multi-Attack. Could do Imp Grapple (esp for the jag), though I'm not sure why we wouldn't just give them Imp Grab.
They've already got Improved Grab. I suggested just for the grapple boost, since most sabre-toothed cats probably wrestled big prey. Could make it a racial bonus instead, I guess, but I prefer the feat.