These aren't terribly exciting, as written. We could probably just make 'em a template to add to a mummy that gives it a bite attack equal to the appropriate animal.
If we want to get creative, though, we could give them some "totem" characteristics of the grafted animal. Perhaps some cunning-related ability for the jackal, added durability for the hippo, stealth and/or grapple for the crocodile, and so on...
Yes, they do need jazzing up, they have identical stats to standard mummies apart from having 2 or 3 attacks and dying if you ressurrect them, which does make them nastier but not terribly interesting.
I'd prefer to use these bare-bone stats as an inspiration for three new monsters. We could certainly slap on some animal-based SA/SQ. I also feel they're mummies sacred to animal gods like ancient Egypt's Sobek (crocodile-headed god), Anubis (jackal-headed god) and Taweret (hippopotamus-headed goddess) so maybe we could add some thematically appropriate supernatural/spell-like powers.
Oh, and I'm thinking we should give them some turn resistance, probably +2 like a spectre.
It'll be easier doing them all at once, although I'd like to keep their final stats separate.
So to begin with, I think they should have at least the 8 hit dice of a standard SRD Mummy, and give them strong differentiation in their stats.
Hmm, we may want to make the Hippo and Croc-Mummies both Large size as well to partially justify their higher stats (after all, they're likely a cross between a Huge hippopotamus or giant crocodile and a Medium human).
The Hippo-Mummy could have incredible Strength, Say Str 30 like an elephant, so it has 2 slams doing 1d6+10 as its primary attacks and a 2d6+5 damage bite as a secondary attack. All with added mummy rot, obviously.

That way it has 2 slams that each do the same damage as a standard mummy (1d6+10 in 3E, 1-12 in AD&D).
The Crocodile-Mummy could have incredible Toughness. Maybe give it a higher DR than a standard mummy's 5/-? It could also be stronger than a standard mummy, just not as much as the Hippo. Say, Str 27 but with sharp claws instead of the hippo-mummy's slams, giving it two 1d8+8 claws and a 2d6+4 bite?
The Jackal-Mummy has the lowest damage so let's say it's only as strong as a regular Mummy and is only Medium sized. It is armed with an ankh, I guess we could treat that as a battle axe (1d8+7 damage) plus a bite for, say, 1d8+3 damage? It needs something to bring it up to par with the other two mummies - some kind of magical powers? We could make it faster (or give it a normal Mummy's speed and say one of its abilities is an
expeditious retreat SLA).