Translation:The dolphin and the whale are mammals.
An example of what language teachers call : un faux-ami: In Greek, η φάλαινα is a whale. In French, un phalène is a moth.
But French for "whale" is "baleine," which is likely cognate with "η φάλαινα."
Right we say "false friend" and they can cause havoc in language learning. Even experienced learns sometimes slip up.
It seems to me that it is more natural in English to translate as: Dolphins and whales are mammals. When we refer generally to species, we usually keep it singular in Greek, but plural in English.