A deer in a dream often represents gentleness paired with heightened alertness -- open and unguarded, yet ready to startle at any moment. Encountering a calm deer can reflect a tender part of yourself you're learning to trust in the open, while a deer that flees may point to vulnerability you're not yet ready to expose. The dream usually invites more patience with your own softer instincts.
To unfasten the tongues Mercury must be in its house and it is efficient with those who Mercury rules. - Picatrix (Andalusia, ~1000.AD)