How to Prune goat's horn cactus?
No pruning required

goat's horn cactus · Astrophytum capricorne
Under glass grow in a sharply-draining loam based compost (JI No2) with extra perlite or sharp sand or a proprietary cactus compost, in bright filtered light. Water thoroughly once the compost has become dry in the growing season from spring to early-autumn, but keep dry when dormant. In warmer climates outdoors grow in sharply drained, poor, slightly alkaline soil in full sun
No pruning required