Do you know His Dark Materials? And what do you think would be Mycroft's soul animal aka daemon?
I do indeed! HDM is a fantastic series, and I love its Sherlock crossovers! I have read and adore this amazing fic series by Etothepii. I dabbled in that world myself sometime ago, with this not very good picture set.
I think Mycroft’s daemon would be a lioness. A lioness is proud and strong, but terrifying and predatory. More importantly, a lion is featured on the Royal Coat of Arms.
I can imagine a young Mycroft expecting his daemon to settle as something rather different - a crow or a raven, perhaps, or some other sign of intelligence. The lioness form came as a surprise and told Mycroft something that he had yet to discover about himself.
Full grown, she hides behind his desk - necessarily large and imposing - while he takes meetings with politicians who have simpering lapdogs, pampered Persian cats or serpents for deamons. Just when they think they have got the measure of Mycroft, or think they have the upper hand, she will stalk round from behind the desk to face them, a growl in her throat. They leave terrified.