Well, this at least he doesn't mind explaining so much, especially if it distracts from topics related to his feelings on things.

"Somewhat. A proper familiar is bound and connected to the mage by contract, and can be used to aid the mage in various ways. Animals are one option, but there are other possibilities as well."

Merlin just happens to be utterly indifferent to what's normally the biggest risk of using a live animal for one: his last familiar was a powerful magical beast to begin with, so clearly a new one becoming that later isn't a concern.