Defenders of Earth

All I can imagine is every animal in an army hat

That's almost the plot of Avatar

Half of them are gonna die from mosquito borne illness, probably.

Our ancestors hunted and killed large animals with sticks. So I think it's unlikely that alien invaders with advanced enough technology to travel through space and wipe out weapon-weilding humans will get killed by animals like hippos just by getting mauled by them. However, immunity to earth's bacteria and animals that are carrier to some sort of diseases like rabbies and malaria are different matter.

I doubt your average human can name a hippopotamus just from a description of it

Don't forget that in War of the Worlds the Martians were defeated by bacteria.

Aids will kill them first