Your question is an excellent discussion piece, as the topic of cultural appropriation is quite nuanced. I will give you my perspective and I hope that you will hear from others to gain a well-rounded understanding.
To answer this, it is important to first address the difference between cultural appropriation versus cultural misappropriation.
Cultural appropriation occurs whenever someone adopts part(s) of a culture that they do not belong to. If done with respect, appreciation and understanding of that culture's values, then cultural appropriation can be a wonderful bridge between cultures that enriches everyone's experience! The diversity of cuisines across the USA is a great example of (positive) cultural appropriation. Learning another language is an example of (positive) cultural appropriation.
When cultural appropriation occurs without respect, appreciation or understanding of that culture's values, or worse, if it occurs with intended disrespect or exploitation, then it becomes an unacceptable example of cultural misappropriation. For example, wearing a native American war bonnet as part of a costume is exceptionally insulting form of cultural misappropriation, because the war bonnet is an incredibly special honor that must be earned. Wearing an "Indian Warrior" costume that you bought for $10, indicates a deep ignorance of Native American ritual and culture.
So if you are worried about cultural misappropriation, ask yourself, did I "earn" this or do I really understand the importance of this tradition within that culture? If you are learning to speak another language or cook an ethnic cuisine, you are clearly earning that ability with dedicated learning and effort on your part.
As for your specific example of a Rasselbock costume... That's a pretty safe bet, because it's a mythological creature that appears in German folklore as a playful oddity (so it's not a serious aspect of German culture). I highly doubt that you would offend anyone who is German or is of German heritage... My only hesitance is that I noticed you spelled it wrong (It's Rasselbock, not Rasslebock), so I wonder how much you actually know about that folklore. If you wear a Rasselbock costume and somebody asked you about it, would you be able tell them much about it? If yes, wear it! If not, reconsider your costume. Or better yet, just read up about it first! Then you can wear it and share your knowledge about that folklore with others. :)
Here's some light reading for you if you need it.
By the way, if you ever have the opportunity to visit Munich, Germany, go see the Museum of Hunting and Fishing! Their taxidermy exhibit has dozens of Rasselbock and Wolpertinger mounts on display.