One of my few unicorns!
As a little foal she was born premature and very very sickly, and was abandoned by her mother because her chances of surviving were near zero. In a final desperate bid for survival, her magic latched onto literally anything around her that was 'alive' and attempted to bind it to her in some kind of tragic symbiosis. And it worked! Her body is a haven for moulds & fungi, and works in perfect sync with these foreign bodies. She's got relatively little growth on her right now, with just the toadstools and little mushrooms growing on her. Patches of mould are constantly growing in her fur and move around all the time.
Sadly, she is very dangerous to be around! She breathes very pathogenic moulds, such as Claviceps purpure (ergotism) and aspergillus spores.