Lake And now forrrr pointing out the obvious. That would make her a bitch, which is a female DOG.

Ah but on the contrary if taken literally "Son of a Bitch" would imply that that Maisy is a male dog and furthermore Neko's gay interspecies lover, a theory rendered void when taking into account the litter of kittens she gave birth to and the usage of the phrase as an exclamation. QED

The little lines are instead of speech balloons, they're not just labelling characters. Maisy's calling the dog a son of a bitch.

It's Funny, "Cus It Happens That way! How Would The Dog Be Born Without A *Bitch*?
*Female Dog, Of Coarse*

It's funny 'cuz it's true. Lol