The truth is that the glow is only the reflection of light from some other source. The reason that reflection takes place is that there is a layer of crystalline substance in the eyes of many animals. This substance has the ability to reflect light. This reflecting layer also helps the animal to see in the dark, which is why they can see better at night than man can.
The difference in the colour of the light reflected from the eyes of the animals is due to the different number of blood vessels in their eyes. An animal that has many blood vessels in its eyes will reflect a reddish glow. If it has fewer numbers of blood vessels, it will have a whiter glow.