The affix ṭac comes after the word uras standing at the end of a tat-puruṣa samāsa when it has the sense of 'the most excellent of its kind'.,
The word अग्र means 'the principal': as the word उरस् is the principal member of an animal, so by metonomy it has come to mean 'the principal'. Thus अश्वानां उरः = अश्वोरसम् so also हस्त्युरसम् \the most excellent of horses or elephants\.
Why do we say \when meaning the most excellent\. Observe देवदत्तस्योरः = देवदत्तोरः \the breast of Devadatta\.,