During the Easter event, the Stormrider Hare was available from certain level appropriate bosses throughout the Spiral. That's the only permanent one that I know of that dropped. There are bosses that drop temp mounts all the time. There are also permanent mounts available from the Crown Shop for gold instead of crowns. They range from 50k-90k gold cost. During the BOXES event you can also purchase recipes for 2 crafted mounts for 60k gold.
Some of the Secret key Bosses drop Perm. mounts. I got a Silver Kirin at the Hollow Mountain secret key boss. That boss also drops the Golden Kirin as well. My partner got it. Also the Secret key boss, Bellosh in The House of Scales drops the Mander Palanquin which I think is pretty cool.