Uther would TOTALLY have an Arcanine, I am all over that. I like the Eevee idea for Gwen, too. Morgana...hmm, something regal but potentially dangerous? The two I have in mind are Ninetails or Lapras - they can be really kind and caring, but probably also have the potential to chew your face off. Arthur, for some reason, I'm seeing with a Cubone. That would be kind of stubbornly loyal, but also hit him round the head a lot when he's being dumb.

(HE NAMES IT MERLIN OBVIOUSLYYYYYYYYY.)

Merlin would totally have a Clefable. That's all I can picture him with. But yes, rofl, he wants to ride that Rapidash with Arthur into the sunset~