*Unearthed Arcana* (UA) 1e page 15 shows Cavaliers can handle and ride other mounts. And the Paladin gains the Cavalier abilities.  

> "4th level a female elven cavalier (and only a female elf) can ride a unicorn as a steed.   
> At 7th level the cavalier can handle and ride a pegasus as a steed.   
> At 9th level, the cavalier can handle and rige a hippogriff as a
> steed.    At 11th level, the cavalier can handle and ride a griffon or
> a similar creature (DM's judgement) as a steed."