אַבְנֵי הַשָּׂדֶה חַיָּה. ייַסִי עַרְקִיי בַּר נַשׁ דְּטוּר הוּא וְהוּא חַיי מִן טִיבוּרֵייהּ אִיפְסַק טִיבוּרֵייהּ לָא חַיי. רִבִּי חָמָא בַּר עוּקְבָּא בְּשֵׁם רִבִּי יוֹסֵי בֶן חֲנִינָה טַעֲמָא דְּרִבִּי יוֹסֵי וְכָל־אֲשֶׁר יִגַּע עַל פְּנֵי הַשָּׂדֶה בְּגָדֵל עַל פְּנֵי הַשָּׂדֶה.
Avnei hasadeh [lit. "field stones," also called adnei hasadeh "grass lords"] is an animal. Yassi from Arke: It is a mountain man and lives from its navel. If its umbilical cord is cut, it cannot live. Rebbi Ḥama bar Uqba in the name of Rebbi Yose ben Ḥanina: The reason of Rebbi Yose (Num. 19:16): “All that touches [a slain person] on the surface of the field”, when it grows on the field.
וְאדני הַשָּׂדֶה, חַיָּה. רַבִּי יוֹסֵי אוֹמֵר, מְטַמְּאוֹת בָּאֹהֶל כָּאָדָם. הַקֻּפָּד וְחֻלְדַּת הַסְּנָיִים, חַיָּה. חֻלְדַּת הַסְּנָיִים, רַבִּי יוֹסֵי אוֹמֵר, בֵּית שַׁמַּאי אוֹמְרִים, מְטַמֵּא כַזַּיִת בְּמַשָּׂא, וְכָעֲדָשָׁה בְּמַגָּע:
Wild man-like creatures are [in the category of] hayyah. Rabbi Yose says: they cause impurity in a tent like a human being. The hedgehog and the bush-mole are [in the category of] hayyah. The bush-mole: Rabbi Yose says in the name of Bet Shammai: an olive's size [of its carcass] renders a person carrying it unclean, and a lentil’s size [of its carcass] renders a person touching it unclean.
Based on these texts, the Adnei/Avnei Hasadeh are wild man-like creatures who dwell on grassy mountains. They have a stem like an umbilical cord which has roots in the ground and, if the cord is severed, the creature dies. It presumably can only walk in the radius of its cord, and is not described as dangerous to people.