SUMMARY—
If you're on a budget and have pop room to spare go with Apes of Set every time. If you're going to reach pop and have the favour to spend the Crocodile wins hands down, regardless of the type of oponent.

Feral does indeed give any summoned or converted animals a +50% attack and +10% hp, +10% hack armour and +10% pierce armour.

Automatic upgrade: Animals of Set recieve an automatic and free +15% hp and attack every age advancement (except the first)

DETAILS—
(Figures listed are for creatures after Feral has been researched.)

- Archaic Age -
ANIMAL	COST*	FOOD	POP*	HHP*	PHP*	AT*
Ape	3	60	2	69.09	62.72	7
Hyena	~	60	2	62.72	67.13	10

- Classical Age -
ANIMAL	COST*	FOOD	POP*	HHP*	PHP*	AT*
Ape	3	60	2	69.09	62.72	7
Hyena	6	60	2	62.72	67.13	10
Gazelle	4	75	2	31.02	24.20	5

- Heroic Age -
ANIMAL	COST*	FOOD	POP*	HHP*	PHP*	AT*
Ape	3	60	2	81.78	62.72	8
Hyena	6	60	2	74.24	67.13	12
Croc	8	112	2	138.06	160.29	21
Gazelle	4	75	2	36.66	28.60	6
Giraffe	7	150	3	54.99	44.46	9

- Mythic Age -
ANIMAL	COST*	FOOD	POP*	HHP*	PHP*	AT*
Ape	3	60	2	94.47	85.76	9
Hyena	6	60	2	85.76	91.79	14
Croc	8	112	2	159.30	184.95	24
Rhino	16	**	2	369.90	394.20	20
Gazelle	4	75	2	42.30	33.00	7
Giraffe	7	150	3	63.45	51.30	10
Hippo	12	225	3	230.10	249.60	20

* COST = favor
  POP = population
  HHP = hit points with hack armor upgrade
  PHP = hit points with peirce armor upgrade
  AT = attack

** I didn't note the food value of a rhino.

Several interesting things can be determined from this data.

MILITARY
I've determined military usefullness by calculating the ratio of cost(favour) or pop to unit power (sqrt(hp*at))

In 'bang for buck' terms the Ape of Set remains the best choice throughout all ages, regardless of whether you're fighting hack or pierce units.

In population cost terms the hyena appears the best choice for the first two ages while the crocodile is the better for the latter two ages. Again regardless of whether you're fighting hack or pierce units.

FOOD
In the first two ages the most efficient animal for food is the Ape with a food to favour ratio of 20:1 as compared to 18.75 for the Gazelle. In the latter two ages the Giraffe is the most cost effective source of food with a food to favour ratio of 21.43 compared to 18.75 for the hippo (and the gazelle).