Yes, fishes is possible and it usually means "species of fish" rather than several individuals.
Trout is one of the most popular fishes to catch.
The most common plural is "fish"
He caught a lot of fish.
There are ten fish in the pond.
We had fresh fish for lunch.
Dear is a term of endearment, usually used by older women to refer to each other when they have been friends for a very long time. If you are with many dear friends, you can say, "My dears, let's go have lunch at the tea room." But if a young person said this, her friends might laugh at her because it is very old-fashioned! It is not a manly form of address, either.
If you mean the animal, the spelling is deer. It is both singular and plural.
A herd of deer grazed by our house at sunrise.