I decided to use heraldry animal meanings for my Merlin daemons because a) it allowed me to know what sort of animals those in that time period would know about and b) I thought the significance of what each symbol means would be influential in how a daemon might decide to settle in a medieval setting. In the HDM books, witches and wizards always have bird daemons, so I've honoured that here, though I've decided it's not a rule in the Merlin verse, as it would be really easy for Uther to pick out who has magic or doesn't.
Merlin-- Sigrun [SEEG-roon] (Old Norse, meaning 'secret lore' or 'victory lore'), a duck. In heraldry, ducks signify someone of many resources, as it swims, flies, and walks on the ground.
Arthur-- Brandra (Old Norse, meaning 'sword'), a lioness."Valiant in courage, strong of body, politic in council and a foe to fear."
Guinevere-- Amandus (Latin, meaning 'worthy of love'), a sheep. Lambs symbolize 'faith, innocence, bravery, gentleness, purity, resolute spirit'.
Gaius -- Slana (Irish, meaning 'health'), a raven. Ravens are symbols of knowledge and of a quiet, settled life.
Morgana -- Delane (Irish, meaning 'dark challenge'), a falcon (that's a common kestel, actually, but they're in the same family). Falcons symbolize someone who 'is hot or eager in the pursuit of an object much desired and did not rest until the objective was achieved.'
Lancelot -- Phaenna [FIE-nah] (Ancient Greek, meaning 'shining'). A deer. Bucks and stags signify one who is a skilled fighter, but will only fight if provoked, but always wins if battle is necessary. It's also considered one of the most 'handsome' charges.
Gwaine -- Jocosa (Medieval English, meaning 'merry, playful'). An otter. Otters represent people who work hard, but know how to play, and live life to the fullest.
Percival -- Iphigenia "Effie" (Ancient Greek, meaning 'strong born'). A deerhound. Dogs of all sorts are considered loyal, strong hunters, and temperate.
Elyan -- Gobnait [GAWB-net](Irish, meaning 'little smith'). A badger. Considered animals who are fierce in fighting in defence of their home. It represents bravery, perseverance and protection.
Leon -- Aalis [AH-lis] (Old French, meaning 'noble sort'). A leopard. "A valiant and hardy warrior who braves dangers with enduring force and courage."
Uther -- Corona (Latin, meaning 'crown'). A crab. Crabs represent someone who grabs onto something and won't let go.
Mordred -- Aranka (Hungarian, meaning 'gold'). A chough. 'Bringer of death'.