Koma, Cosmos Serpent

You can't sacrifice Koma itself to activate its last ability, but you can sacrifice any other Serpent you control, including Koma's Coil.

If you choose the first mode of Koma's activated ability, you can choose a permanent that's already tapped as the target. The effect stopping its activated abilities will still apply.

A spell or ability that counters spells can still target Koma, Cosmos Serpent. When that spell or ability resolves, Koma won't be countered, but any additional effects of the countering spell or ability will still happen.