I heard once that what Rashi talks about in Bereshit זאת הפעם: מלמד שבא אדם על כל בהמה וחיה, and that from these relationships came beings with the form of a man with an animal. I want to know the source of this.

ב"ה Shalom You heard correctly. Rashi indeed says this in his commentary to Bereshit 2:23. The source of Rashi is the Gemara in Masechet Yevamot 63a. If this saying is to be understood literally, is a discussion among the commentators. Many do not understand this literally. (Maharsha, Ben Yehoyada and others. ) All the best