This is in a book. The author is using words as a form of expression and description in his literary prose.
In this instance the words 'cocker spaniel' were used an adjective. Colloquially used, 'correct,' standard, or otherwise the communicated effect is that of a metaphor, or a description of how the eyes looked. They are simple words in the work to describe the eyes in a literary format.
The same effect would be in example:
His beady eyes darted back and forth from the two people in front of him. He looked about to run.
"Beady" is an adjective added to English because authors continued to use it in a literary sense to not mean 'his eyes were made of beads' but that 'his eyes were small, bead-like, and shiny.'
Some reference would be here: http://www.finedictionary.com/beady-eyed.html