Shiva gave Pashupatastra to Arjuna but who possessed it besides those two?
While providing Pashupatastra to Arjuna Lord Shiva says that neither chief of Gods nor humans know of it. So, it is not possible that any other human would have Pashupatastra. From Vana Parva of Mahabharata:
भगवानुवाच | ददानि तेऽस्त्रं दयितमहं पाशुपतं महत् | समर्थो धारणे मोक्षे संहारे चापि पाण्डव || १३ || नैतद्वेद महेन्द्रोऽपि न यमो न च यक्षराट् | वरुणो वाथ वा वायुः कुतो वेत्स्यन्ति मानवाः || १४ || न त्वेतत्सहसा पार्थ मोक्तव्यं पुरुषे क्वचित् | जगद्विनिर्दहेत्सर्वमल्पतेजसि पातितम् || १५ || अवध्यो नाम नास्त्यस्य त्रैलोक्ये सचराचरे | मनसा चक्षुषा वाचा धनुषा च निपात्यते || १६ ||
Bhagavan said, 'O powerful one. I will give to thee that favourite weapon of mine called the Pasupata. O son of Pandu, thou art capable of holding, hurling, and withdrawing it. Neither the chief himself of the gods, nor Yama, nor the king of the Yakshas, nor Varuna, nor Vayu, knoweth it. How could men know anything of it? But, O son of Pritha, this weapon should not be hurled without adequate cause; for if hurled at any foe of little might it may destroy the whole universe. In the three worlds with all their mobile and immobile creatures, there is none who is incapable of being slain by this weapon. And it may be hurled by the mind, by the eye, by words, and by the bow.'" [VanaParva Chapter 60]
So, when astras like Pasupata are mentioned in other places it should be understood as weapons presided by Bhagavan Pashupati and not this powerful Pashupatastra!
Among the divine, Sri Lalita Tripurasundari also possesses this weapon Mahapashupatastra and she used it to destroy Generals of Bhandasura. It is described in Lalitopakhyana of Brahmanda Purana. This is also captured in Lalita Sahasranama as:
महा-पाशुपतास्त्राग्नि-निर्दग्धासुर-सैनिका ॥ ३२॥
She who burnt Asura Sena using MahaPashupatastra
But it is not a surprise that Devi would also have MahaPashupatastra as her form is considered as "शिव-शक्त्यैक्य-रूपिणी" / "Shiva-Shaktyeika Rupini" ie. "Union of Shiva and Shakti in one form".
Drona possessed Pashupatastra and he even used it against Arjuna in the Mahabharata war.
When Drona failed to gain any ascendency over the son of Pandu, the son of Bharadwaja, that warrior acquainted with the course of all weapons, invoked into existence the Aindra, the Pasupata, the Tvashtra, the Vayavya, and the Yamya weapons. As soon as those weapons, issued from Drona's bow, Dhananjaya destroyed them quickly.
(This is also mentioned in the BORI critical edition of the Mahabharata. It is in volume 6 of the translation of the critical edition by Bibek Debroy.)