Hypothetically if Selves can grow enough thick moss to engulf other rocks they could start to move those around, too- hence the possibility of a golem body. However, their usual single-rock, minimal effort body plan is already too big to be very practical, so they wouldn't maintain such a form for daily life.

Rock!Selves is also an Undertale OC with an RP/askblog if you wanna check it out!! Come at me! It's right over here: http://underselves.tumblr.com

Selves and art of Selves (c) Mythee