Rellor began rolling their iconic balls from birth, and continue until evolution, which occurs when said ball finally achieves stability. Although they soon learn the value of packed mud for holding the ball together, and naturally live in muddy environments, the so-called "mud balls" invariably contain whatever is in front of them, and can wind up as lumpy as a Graveler. With little in the way of natural consistency, and weak and undeveloped psychic powers, the Sisphyean challenge with which these pokemon are faced, whether uphill, downhill, or over flat surfaces, is holding the thing together.
Rellor are not the only bug pokemon in Paldea known for carrying around balls, and have developed a fierce rivalry with their counterpart of Tarantoula. A contest between mud and silk invariably favors the former, and a silk core would do an awful lot to strengthen a Rellor ball, so Tarantoula instead resort to the sort of preemptive strikes for which Spidops are famous; a fainted or eaten Rellor rolls nothing.
Humanity tends to eschew Rellor, considering them, not without cause, to be filthy pokemon who spread disease. It is not true, as has been often imagined, that Rellor balls are made primarily of fecal matter, but neither do they avoid picking it up, nor do they avoid anything else potentially hazardous to the health of others. Rellor have strong immune systems and are ignorant of the problems of larger pokemon; they are not poison pokemon, but their trainers take many of the same precautions. Only once they evolve to Rabsca does the ball finally stabilize; for their trainers, this is an occasion for parties where they give away their sanitary equipment!