In one of my own rpgs I had battle birds listed as a cheap alternative to a horse.
Great minds think alike. Our current campaign has large flightless birds used mainly as mounts for barbaric surface dwelling Orcs. The PCs have aquired some, along with training, from a somewhat "friendly" orc trader. The birds are tough, mean, omnivorous, and until they bond with their owners - they sometimes try and take bites out of their "masters". Kind of like a cross between big bird and Oscar the grouch with the potential to severe limbs with the right crit. The PCs love them!
Riding snakes, though, hmmm... Snakes don't seem like they would take you far before the cold blooded critter ran out of steam, and yes, they would see the rider as a "backpack" lunch. Training a snake would require more than a few open ended roles using animal handling, and I still think that their little reptilian brain would ofter revert back to pure instinct - often at the worst possible moments. Rolemaster was first with the riding snake thing, but I always looked at that critter with a boars head and thought it must have been one of the last designs of the night by a small group of very tired, and slightly drunk, ICE employees.
