The following sentence is taken from 'Why the future doesn't need us' by Bill Joy.
South America, like Australia today, was populated by marsupial mammals, including pouched equivalents of rats, deers, and tigers.
How do you account for the use of 'deers' here
I think it's an accident. The examples are small and harmless, medium-sized and harmless, and big and scary. And because 1 and 3 in the series had an 's', 'deer' got one. I think if the order had been different, the s wouldn't be there: 'South America, like Australia today, was populated by marsupial mammals, including pouched equivalents of deer, rats, and tigers.'
Anyway, it's not a conventional exception.
Can we use it to denote different species of deer as in the case of the word 'fishes'?