Fish
Yes. All fish are multicellular.
Fishes
no
I guess nothing, it is common for people to eat more than one fish a meal if they're small
The plural of fish is fish or fishes, both are valid.
more than one lynx is call lynxes
you call them possums if your talking about more than 1
no they have only one
More than one fisherman is fishermen
More than one platypus is two or more platypuses. The plural is not 'platypi'.
swans
Ants