Well, if they're a size larger than regular mummies I guess it makes sense for them to have double the Hit Dice.
Here's an idea: I'm thinking these creatures are melee monsters, a role that core Undead are not very good at. But these things are emblematic of their gods, so what it they are under a continuous effect similar to the
divine power or
righteous might spell? The former would give them full BAB, the latter would explain why they are Large sized despite their parts mostly being human. I'll rough it out as so:
Might of the Gods (Su): A composite mummy constantly benefits from an effect similar to both the divine power and the righteous might spell. This gives the Mummy a Base Attack Bonus equal to its Hit Dice, a +6 enhancement bonus to Strength and a +2 enhancement bonus to natural armour, +1 hit points per Hit Dice, and increases its size by one step (to Large).
All of these bonuses are included in its statistics above. Should the composite mummy enters an area under the effects of an anti-magic fieldhallow dedicated to a god opposed to its patron the mummy loses all its Might of the Gods benefits, changing its statistics to the following:
*STUFF*
The composite mummy's patron god can suspend its gift on the mummy whenever it wishes (effects as above) to punish its creature for some misdeed or blasphemy. The god can also remove the Might of the Gods quality from a composite mummy that has entirely lost its divine favour, which causes the mummy to be immediately destroyed.
That would make the Croc Mummy HD 16d12+16 (120 hp), AC 23 (-1 size, +14 natural), BAB +16, Str 30 (higher than the current Str 27).
Without the benefits it's Medium-sized, HD 16d12 (104 hp), AC 21, BAB +8, Str 24 and has its weapon attacks downgraded.
Oh, I also like the idea of giving the Croc Mummy a slightly higher Dex, since both a regular and giant crocodile have Dex 12.
How about Dex 13, to give it some more feat options? That would also increase its AC by a point.
Oh, and what about my earlier suggestion that the Croc Mummy has a spell-like ability to summon crocodiles (regular or giant) and water elementals?
EDIT: By the way, I've changed my mind about making the Jackal Mummy Medium-sized. I quite like the idea of a 12-foot Jackal-headed Undead, so let's make them Large as well, but with a lower Strength bonus from Might of the Gods - say +4 like
righteous might gives, so they get Strength 28.
Oh, and the Hippo Mummy could get a higher Strength bonus from Might of the Gods, say +8 or +10, giving it a higher Strength than the 30 I proposed earlier. I like +10 for Str 34, equal to a combination of both the +4 size from
righteous might and the +6 enhancement from
divine power.
EDITED EDIT: I remembered the base Mummy has Str 24, not the 20 I used in my original draft of this post, so I re-tweaked the proposed Strengths.