The Kitzur shulchan Aruch siman kuf ayin vuv, paskens, since we are not experts in how chazal defined “rachin” and “kashin”, we should always be machmir (I’m assuming others have some familiarity with the sugya by using these terms). Do you plan on sitting on this rug? Walking barefoot on this rug? These factors might come into play when you ask your Rav this shayla (if it turns out the rug contains shatnez).
“why is shatnez a problem you aren’t wearing it?”
Midirabanan it is. the braisa of “esser metsiyos” is brought down in a number of places. See Beiya yud daled amud beis near the bottom, for example.