Because between hell hounds, howlers, yeth hounds, and death dogs there weren't quite enough supernatural evil canines.

Also, these beasties are terrible. 24 RHD wouldn't be great even if it was Outsider, but it's Magical Beast instead of that. Stats are high, but not high compared to what a typical Epic creature gives. One Bite (4d6 with 19-20 crit range and some slight acid) which is probably best used for a Mouthpick weapon. SR 17 is hardly relevant at the levels where these monsters are meant to be faced, let alone ECL 24.

A problem is that this bite is specifically required for Improved Grab and Charging Bite (swallow whole by any other name), which means that those can't be used while using a weapon. It's possible to get around that issue, but the abilities themselves aren't great at level 24.

The only redeeming factor here is the free action breath weapon, which I suppose could be interesting if combined with metabreath feats. Maybe.

-0 LA: outclassed in basically every way by something else.

Elite Demon War Mount

Six more RHD? Sure, why not. The only thing worth noting is that it's now proficient with armor. -0 here as well.