Oh lets see...I have a LOT of these. x'D
I've seen 'clone' characters, crazy neon mate-centers in Bonfire, and all manner of random animals. My friend AlphaEclipse and I were even rp-attacked by a big group of strange looking wolves. The second one was kind of creepy...
Another thing I've seen are tons of Hetalians in Ascension. Every day I go there, there's always a group of Hetalians. It's basically become a hangout for lots of Hetalians. xD
I've seen dragons and griffins in Sky's Rim as well, and horses in Bonfire, both numerous enough that they form rp groups. You see something new and interesting (or creepy, like those weird wolves) in FH every day.