Is this an example of assonance: How now, brown cow?
Let's first talk assonance - it's the use of the same vowel sound with words close together to create noticeable repetition (there needn't be rhyming, merely repetition).
Does How now, brown cow meet the definition? There is a repeated vowel - "ow" - and the words follow each other, so there is no question the repetition is noticeable.
And so yes, this is an example of assonance.