That IS a good question... since one cannot go by simply mass of the two..
But one must look at the energetic values attached to the chinchilla versus the goat...
I think...due to the fact chinchillas are quite active...and can build up static elec. all on their own very quickly...
harnessing that ..I would guess..the ratio for output would be something akin to..
20 hamsters on the wheel are the equivalent in elec output to say...
12 chinchillas...
whereas ..goats are fairly calm creatures...
so one would need either young goats ..sporting...
or mating season...
so there is limited output..
So the ratio would have to be much greater than hamster or chinchilla ...to balance out...

I would think also that the goats may indeed start trying to chew and digest the equipment and harnesses ...thereby threatening electrocution...

Actually...
all the test subjects...
hamster, chinchilla and goat...
are very likely to die from electrocution from eating the wiring...
and quite possibly either start fires or catch fire themselves in the process...
the carnage and confusion would be astounding...

Unless...
One could minimize the amount of wiring ..externally...
Say...
Qi-charging-hamsters?
but in reverse...
So they "feed" the Qi charger...

It does have a mad merit.