Vrisha is the bull, and in the religious landscape the hatha texts came out of the bull is Shiva's. Nandi is his mount and his gatekeeper, and a Shaiva temple sets a carved bull facing the shrine so that the animal looks toward the god through the doorway. Devotees pass it on the way in. It is among the most familiar sculptural presences in South Asian religious architecture.
The hatha corpus is Shaiva in its framing. The Hatha Yoga Pradipika opens by saluting Shiva as the teacher of hatha yoga, and the tradition it comes from traces its own transmission from him through Matsyendranath and Gorakshanath. A bull posture in that setting is not a randomly chosen animal in the way the locust or the frog might be.
The manuals do not say so, and the atlas does not assert an intended reference, because a name shared with a familiar figure is not evidence that the figure was meant. What the entry records is the description the Gheranda Samhita gives, the animal the name denotes, and the religious world in which that animal already had a settled place. Readers can weigh the connection themselves; the atlas marks where the sources stop rather than filling the gap on their behalf.
The bull is not the only Shaiva presence in the posture vocabulary. The thunderbolt is Indra's weapon in the older material before it becomes a Buddhist ritual implement, the lion seat is the throne of an enthroned deity, and the lotus is the flower on which gods and Buddhas are shown seated. The naming stock of the classical list is drawn from the same iconographic world the practitioners lived in, without the manuals ever pausing to say so.