Dogs, horses [...] are animals that are found on land and are called mammals.
Octopuses, sharks [...] starfishes and goldfishes are animals that are found in water and are called fishes.
Fish is countable when speaking of the animals; the plural is normally the same (one fish, two fish), but fishes is also a plural, and is used mostly when speaking scientifically of different kinds.
When speaking of food, fish is an uncountable noun. 'We ate a lot of fish yesterday.'