I just finished translating the 1812/15 first edition of the Grimms “Kinder und Hausmärchen” (KHM) so I have a few ideas.
Here are some of my thoughts (and in no particular order).
The Grimms differentiated between “Märchen” and “Sagen” (legends). Their “Deutsche Sagen” (DS) were published after the KHM and contain short texts of German legends – i.e. things that supposedly really happened at some time and at a particular place. These were different from Märchen because they took place in actual real word locations and at specific times. Märchen on the other hand, are not linked to any particular time or place. Donald Ward translated the DS into English many years ago.
Jacob Grimm also published books titled “Deutsche Mythologie.” Fortunately James Stallybrass translated these 4 books into English in the 19th century. English language readers can read the English translation of Jacob’s Deutsche Mythologie in the translation “Teutonic Mythology” by James Stallybrass -Vol I, Vol II, Vol III, Vol IV.
If a “creature” can also be a character, then one should look at 1815 #4, 5, 6, 7
The order of the texts in the KHM is not haphazard or random. The four texts - #4 Regarding a Young Giant, #5 Dat Erdmanniken, #6 The King From the Golden Mountain and #7 The Raveness (Raven Girl) - were also placed together by the Grimms because they were of the opinion that all of the texts related to the Siegfreid Saga. The young boy in #4 is Siegfried. The boy, like Siegfried, hits clumsily when working for the smith (Reign). In #5, they say that: “The earthmanlet is Euglin and Alberich, whom the hero also makes through force predisposed to him.” In #6 they note that: “The agreement with Sigfried first starts, where the youngster like he is driven away upon the water. The king’s daughter, that he frees, is after the German Saga Chrimhild upon the Drachenstein, but otherwise, especially after the Nordic Saga, Brunhild, because for Gudrun (that is Grimhild) he does there, as in the Niebelung Lied, nothing. The dragon, who holds her captive, appears in the way, that she herself is transformed into a snake.” They also note the similarity of the becoming invisible with the “Tarnkappe” [cloaking-cap] in the Nibelungenlied. “In his relationship to the queen also shines that of Brunhild through, she knows, as in the Nordic Saga, that he will become un-happy, when he goes away from her, and her connection to him has something secret.” In #7, they note that: “The golden palace on the Glasberg is the Flame-hall [of] the Nordic saga almost concurring with the old Danish song of Elskovsviser … where Bryniel sits upon the Glasberg; which only a special horse (Grani) is able to climb. The relation and the switching of the flame and the shimmering Glasberg lies very close. – The sleeping-potion, from which she warns him and which overpowers him, is the forgetting-potion [of] the Nordic Grimhild.” All of this is more completely described in the Anhang, the one that most American and English translators (apart from myself and Margaret Hunt) do not deem important enough to translate completely. #50, #51, #52, and #53 were also placed together because they were all Low German Dialect texts. Other texts such as #61 and #62 were also placed together because they dealt with God, Jesus and imitation. Hunt’s translation is available at: http://www.archive.org/stream/grimmshouseholdt2grim#page/n7/mode/2up.
It is in the Anhang that the Grimms discuss the origins of the texts, etc.
KHM 1815 # 8, “The Clever Farm-Daughter” contains a trace of the sage of Aslaug, daughter of Brynhild and Sigurd.
Here an excerpt of my translation of the Anhang entry to the 1815 #22. “Hans my Igel”
“ 22. Hans my Igel.
(From Zwehrn.] Is king Porc from Straparola (ii.1.) but here better, [more] fantastical and original, only Hans should have told a king the way and [was] deceived, in that he first, as by Straparola, in the third time is released. Igel [hedgehog] Stachelschwein [thorn-pig] und Schwein [pig] are mythically one, as Porc and Porcaris; below in another also good depiction it is a donkey (no. 58.). These two Märchen make with No. 1. and 68. in the first volume and No. 2 13 41 in this [one] a row [of] related [tales], upon which still others not so related attach, compare the there found Anmerkungen. Regarding the basic idea see an Anmerkung to the altdän. Liedern. P. 528. 529.
People, which impetuously implore God for child-blessings, are often punished in Märchen with such fail-births, that they hereafter, when the parents are humiliated, still transform into humans. – The return of the child into the paternal house is as in the Young Giant in No. 4.
Igel = hedgehog.
Wilhelm adds a note: “the Finkenritter rides as Hans upon a rooster.” Rölleke states that this refers to “Der Finkenritter, ein Volksbuch, Straßburg around 1560.””
Other creatures? Talking frogs, cats, horses, the devil, wights, elves, lions, birds, donkey, lamb, fish, the fox of course, crows, giants, etc., etc. All are probably also found in German mythology. Ands we can't forget about dragons and conversations with them. Consider the dragon Fafnir, the eating of the heart and understanding the speech of birds. Tolkien's "conversation with Smaug" also reminds much on Sigurd's conversation with the dragon Fafnir. Knowing someones name also give one power over them - think of Rumplestiltskin, etc.
All in all, the Appendix to the KHM and Jacob’s “Deutsche Mythologie” will probably explain much of it.
Lastly, I would not limit the question to creatures only. Much of what is found in the KHM can also be found in myth. For example (and in no particular order) - the cutting off of horses heads (Falada) -horse heads were considered to protect from enemies. Some houses today still have horse heads on their gables. That speech continues to live in Falada's head is as with Odin and Mimir who councils him after his head is cut off. The arguing of giants regarding the division of treasure, etc., etc. Characters often go over or into water to reach the "other realm," the "other world." This might go back to the idea that the realm of the dead was underwater at the bottom of rivers, lakes, and oceans. Frau Holle can only be reached when the girl falls down into the well. The Undying Lands, home of Tolkien's elves is also reached by going "over water" as I recall.