Is not so fast countering Beast's Bellow effect on their own unit or the total effect?

When you use Not So Fast to counter the triggered ability of Beast Below, it counters the entire ability.

Ruling

According to RiftJudge FAQ #4968, when countering an ability with multiple effects or targets, the entire ability is countered as a single item on the chain. You cannot selectively counter only one portion of the ability.

Sequence

1. Beast Below is played as a unit.
2. Its "When you play me" triggered ability goes on the chain.
3. You play Not So Fast in response, targeting the ability of Beast Below.
4. Not So Fast resolves, countering the entire triggered ability of Beast Below.
5. Because the ability is countered, it does nothing (Rule 412.1.a).

Note: While the ability is countered, the unit Beast Below itself remains on the battlefield, as it has already entered play. Countering a "When you play me" effect only negates the ability, not the unit card itself.

Note: I could not fully verify this ruling against the retrieved rules.