I voted NO because here's why:
1. Opening up characters to established franchises can create linked continuities
With the "MLP" example: If they exist, does their homeland? Where would it be in relation to Lismore? Would they be an established "race" in other places as well? What is their history? One could argue that "that can be applied to any character," to which I'd have to give the response that the bipedal humanoid with animalistic traits tends to be under the blanket of "furry," whereas "MLP" is not and should not be, while Sonic falls in the same category.
2. Character physiology
Cartoon characters are renown for not following conventional principals for their avatar. In Lismore it's been pretty simple: Animal-like avatar based on the human biped model. Unique features? Yes. Ears, tails, digitigrade feet, and other animal anatomy, but they are all generally on the same musculature and structure of humanoids. Cartoon characters don't require that, as referenced with the "MLP" example, they can be quadrupedal talking cartoon horses. Or sonic characters with gigantic heads and stick arms. How would these things logically exist in the same world that a human would? How would they have logically evolved to the point they're at? "BUT BEA!" I hear you ask, "THEY COULD BE MADE BY MAGIC!" To which I'd reply that if they can be created by magic, so could a giant pony-eating dickbeast. "WHAT THE HELL IS A DICKBEAST?" A race I just made up via magic.
3. You have no moral grounds with which to deny even more obscure characters
So, my little pony? Sonic characters? Where would it end? Would there be a limit? Can I play an oompah loompah? He-Man? He man had some pretty technological "magic" things, would those be canon too? Come to think of it, I've heard "my little pony" even had machines in their series. Sonic's primary antagonist was basically all about machines and robots. "BUT BEA! I hear you squeal, "THE MAGIC! THEY CAN BE WILLED INTO EXISTENCE WITHOUT ALL THAT STORY STUFF!" To which I would have to say DICKBEASTS. >8C
4. Copyright.
According to the LL ToS you aren't supposed to sell copyrighted material without the express consent of the IP's original owner. <- This right here is why we didn't have Sergal avatars until recently. Originally there was a sergal avatar in production by one Naoki Dengaku and in its initial trial run, the creator and IP owner of the sergal caught wind of it and said they would work to have it removed from SL permanently. In a fit of rage the one making the avatar complied by not selling it, but rather handing out the original sculpts full perms for free to anyone who asked. This was years ago and we didn't see a Sergal avatar until KZK cut a deal with their creator. I haven't done the research but I strongly doubt the creators and IP holders of MLP, nor Sonic Team/Sega, nor Mattel have anything to do with any of the SL variants of their intellectual properties. They are just kind enough/fat and lazy enough not to go on the hunt for it and wipe it from SL in a similar situation to the Sergal incident from before. So, by technicality, their very existence is possibly against the LL ToS.
5. Ruining player immersion
The idea of playing a role involves you getting into character, playing someone who is not really a digital copy of you but rather a character you manifest to play in a world or story. It's a little jarring when you suddenly find your fictional character in this unique world suddenly joined by Sonic the Hedgehog with Knuckles waddling up the wall beside you while he-man orders a milkshake from the bar and a "My Little Pony" is busy doing a jig at your feet and then A GIANT DICKBEAST COMES CRASHING THROU- you get the idea.
In conclusion: Vote yes for dickbeasts.
-Bea