"Necromantic experiments in the production of artificial undead intelligence has thus far resulted in the creation of the hybrid flying serpent-rat. This hybrid, we have found, possesses astonishing survival and procreation instincts, even in the most unsuitable of circumstances. Thankfully, these flying mutants are easily tamed and trained." - From "Necrarium Tomeus" by Necromaster Barbaross the Deceased.
Common recon unit
Scout: sight range +1
Extremely cheap and quite fast flying scouting unit. Build a few of these to uncover the map. Low cost has its drawbacks though: this unit will fall to any semi-decent archer or fort on the way as it has abysmal hit points and no resistances to speak of.
Bats can be a cheap and effective way to destroy unprotected enemy transports while still on the water.