What is the common characteristics of Tiger, Dog, Rat and Dolphin?

They're all placental mammals:
young are gestated within a placenta

They're all mammals:
young are fed milk which are secreted from specialized gland(s) that are located on the ventral surface of the mother.
Hair or fur (note: with dolphins this is reduced to a few "nose hairs" around the blowhole)
Warm-blooded
Four-chambered heart
Three small ossicle bones in the middle ear

They're all amniotes:
Young are gestated within membraned structures

They're all modified tetrapod vertebrates:
Four limbs (with dolphins, the hind legs are internal vestiges, as are the phalanges of the forelimbs)

They're all vertebrates.
Backbone made of vertebrae

They're all craniates:
As adults, they have a head with a skull

They're all chordates:
At some point in the life cycle, post-anal tail
At some point in the life cycle, notochord
At some point in the life cycle, pharyngeal pouches
At some point in the life cycle, dorsal nerve cord

They're all deuterostomes
The blastopore does not become the mouth

They're all bilaterians.
Bilateral symmetry at some stage of life

They're all Eumetazoan animals.

They're all eukaryotes.
Cells have a membrane-bounded nucleus and membrane-bounded organelles

They're all living organisms in superdomain Biota.
Life is cellular or postcellular
Information storage is in nucleic acid(s)
Functionality is through proteins
cell membranes are phopholipids with embedded proteins

They are all vertebrates, and mammals, and care for their young for a time before a certain level of maturity.

the most common thing is that that they're all mammals.

Dolphin And Tiger