Mounting a Jaguar can be fatal.

It's like this forever since the classes came out, and I don't think this will ever be fixed... the hitmodels are WAY too large for both classes...

Yeah...
From what I've noticed your hitbox size is basically as follows: If you draw a square over your character, the smallest the square can be while encompassing your entire character, that's your hitbox. Mounts, Mechanics, and Wild Hunters suffer because of this. It's really bad on any mount that's gigantic. It's really noticeable on my Paladin in JMS, where I have the free MSDS Cloud Mount. The hitbox on that mount is astronomical in size, and as a result if I use it I can't get anywhere without taking a lot of hits.

It really sucks, but there's not really much they can do about it, and it's unlikely they will even try.