Arfur (atys)
I asked about merchandising last year but got no reply. The situation has changed since the early days. Ryzom is now open source so I assume the logo, images etc. are also open source (i.e. free to use). Please correct me if I am wrong.
You have to be kind of careful here. The artistic assets are CC-BY-SA which provides you rights to use or modify them so long as you attribute Winch Gate and re-release your changed assets under the same license. Another player on the forums kind of suggests this with an over simplified explanation of what "open source" is.
However this does not indemnify you from violating Winch Gate's trademarks. So you can download the yubo model and textures and use it in your game but using the Ryzom logo would be a violation of trademark if not done with Winch Gate's permission or used only with regards to "fair use" which typically pertains to situations such as writing an article about the game, etc.
If you were going to be doing something that was obviously Ryzom merchandising you would be best served by getting Winch Gate's blessing first to avoid any legal quagmires associated with the differences between copyright and trademark law.