What kind of creature is Yoda?

a made up one.

Indeed a made up one, but if you’re doing a little bit of “pretending”, even the Star Wars’ Databank doesn’t have any information on Yoda’s race. So there you go, there really is no answer.

He was supposed to have been very long-lived (900+ years) and was comfortable living and eating in a swamp. The ears suggest some sort of either high-frequency hearing or possibly evolution in a poorly-conducting acoustic medium (very thin air?). The small stature suggests a high-gravity environment, possibly a slightly larger planet than ours. He breathes the same stuff we do. His food is palatable to us. He has binocular, stereoscopic vision. His green pallor probably indicates something about his natural habitat, but I have no idea what. Is there a biologist in the house?

Yoda’s race has never been revealed (or determined). There have been a couple other characters in the Star Wars universe who shared Yoda’s race, but not much is known about their origins.

Yoda is portrayed as a wise and powerful Jedi master in the fictional Star Wars universe. George Lucas originally wished Yoda to follow his other characters in having a full name: Yoda Minch, but instead opted to have many details of the character’s life history remain unknown. Yoda’s race has never been stated in any media canonical or otherwise and he is merely stated to be of a “mysterious species” by the Star Wars Databank. Often his race is given as “tridactyl species”. It is presumed that he is the same species as Yaddle and Vandar Tokare given the strong visual similarities: Tokare appears almost identical to Yoda in depictions of the character.[original research?]

Early in the development of Episode IV, George Lucas included a species called the Whills, a distant, omnipresent race who were to take the story-teller perspective. It has been postulated by fans that Yoda, Yaddle and Vandar Tokare are Whills; recently, there has been passing reference to a Shaman of the Order of the Whills in the novel adaptation of Star Wars Episode III: Revenge of the Sith, confirming their continued canonical existence in the Star Wars universe. However, George Lucas has firmly denied that Yoda’s species is in fact that of the Whills.
wikipedia

He is a member of the Whill Species. The only other one I know of is Yaddle, who isn’t technically part of SW canon because she was a Jedi Council member in the SW: Tales comics series.
EDIT: looks like you beat me to it, Perchik…sorry

It is completly uknown. no book (that anyone i know knows of) contains his homeworld or species. possibly Georg just wanted him to be a complete mystery and all those expanded universe authors wanted to keep that going.

Read the synopsis for star wars episode ix and in the third paragraph it reveals yoda is a whill and his home planet is Grentarik :

Luke Skywalker, Ben Skywalker and Anakin Solo are on Grentarik, the home world of Yoda the Whill, looking for information about the dark side of the Force. The three Jedi locate Yoda’s old dwelling and begin to search through Yoda’s belongings. They are looking for clues as to where they can find definitive information on how to destroy the dark side. Anakin finds a piece a data pad that gives details that at the central Whill library, the three can find out where several ancient Sith holocrons may be hidden.