i have heard people suggesting school mounts such as seraphs and helephants i mean what the heck how to you ride those. well i have some better ideas.
pyromancers. a phoenix since riding on a helephants shoulder is dumb and fire dragons already exist.
thaumaturges. frost beetle since again riding on a collosus shoulder is a bad idea.
diviners. storm shark since most of us know shark mounts are coming.
theurgists. centaur since unicorns or dream fillys already exist
necromancers. none of the death spells are ridable so it has to be a mutation of a different spell maybe a danger hound
sorcerers. a hydra since theres no way to ride a judgement.
For pyromancers you should ride a heck hound. For storm a kraken. For ice an ice wevrin.(sorry if its spelled wrong) For balance a scorpion. For life a centar. For death a giant vampire of danger hound. For myth an ortherous.