As I know, when Arjuna did penance to Indra, Shiva referred to the pashupata weapon as "roudra" but also as a pashupata, implying that it is another name of the weapon according to Shiva:
'"O great king! Having done this, the illustrious one discarded the form of a kirata and appeared in another divine form, dressed in a wonderful garment. The illustrious god himself, with the bull on his banner, 164 yellow-eyed and capable of assuming many forms, the wielder of the pinaka, 165 could be seen in his own form, accompanied by Uma. O scorcher of enemies! After the battle, I stood there, and the wielder of the trident told me, 'I am pleased with you. 'Grasping the bow and the inexhaustible quivers that never run out of arrows, the illustrious one returned them to me and said, 'Ask for a boon. O Kounteya! Yo soy satisfied with you. Tell me what I can do for you. Or brave one! What is the desire of your heart? Tell me and I will grant whatever is in your heart, as long as it is not immortality. ' Then, I joined my hands in salutation, with my mind set on obtaining weapons. I bowed to Sharva167 and spoke these words, 'If the illustrious one is pleased with me, I ask for this boon I wish to know about all the weapons that the gods possess. 'The illustrious Tryambaka168 told me, 'I will give. O Pandava! My own weapon, roudra, will always be present before you. 'Satisfied, the Lord granted me that pashupata weapon.169 Having given me that eternal weapon, Mahadeva told me,' This should never be used against humans. OR Dhananjaya! This powerful weapon should only be used if you are hard-pressed. It can be used to counter all other weapons. 'Because of Vrishadhvaja's favors, that divine weapon, capable of countering all other weapons, the destroyer of all enemies and capable of annihilating the armies of enemies, unassailable and impossible for gods, danavas and rakshasas to endure, stood personified by my side. After obtaining his170 permission, I sat down there. In my very sight, the god disappeared. "'
And in fact Arjuna used that weapon against the kalakeyas:
'"In that battle, with all my showers of arrows, I was not able to oppress them. But they oppressed me. Thus hard-pressed by many who were accomplished in weapons and were skilled warriors, I felt pain in that great battle. A great fear took hold of me. In the battle, I bowed down to Rudra, god of the gods. Saying, 'May there be welfare to all beings,' I used the great weapon famous by the name of roudra, capable of destroying all enemies. I then saw a man with three heads, nine eyes, three faces and six arms, with hair blazing like the sun and the fire.206 O scorcher of enemies! There were giant serpents with flaming tongues on his head. O bull among the Bharata lineage! On beholding that terrible and eternal roudra weapon, I lost my fear and attached it to Gandiva. I bowed in obeisance to the three-eyed and infinitely energetic Sharva.207 Or descendant of the Bharata lineage! I discharged it, to defeat the lords of the danavas. O Lord of the earth! As soon as I discharged it, I assumed thousands of different forms everywhere-deer, lions, tigers, bears, buffaloes, serpents, cattle, elephants, marsh-deer, sharabhas, bulls, boars, cats, hyenas, ghosts, bhurundas, 208 vultures, garudas, sharks, 209 pishachas, yakshas, haters of gods, guhyakas, nairritas, large fish with mouths like elephants, owls and masses of fish and tortoises, all brandishing many kinds of weapons and swords. There were yatudhanas, 210 wielding clubs and maces. There's were many other beings in different forms. They filled up the universe when that weapon was discharged Those many different forms-with three heads, four tusks, four faces and four arms-devoured the flesh, fat and marrow of the danavas. They continuously killed the ones who had gathered there. Or descendant of the Bharata lineage! In an instant, I also killed the danavas with arrows that were destructive of enemies, as hard as the vajra and as radiant as lightning, blazing like the sun and the fire.
But anushasana parva describes the very different weapon:
This was the great and extremely terrible divine weapon, the Pashupata. It was unmatched, impossible to describe and fearful to all creatures. It was gigantic in size, with sparks, and seemed to spout out fire. It possessed a single foot and giant teeth. 156 It had one thousand heads and one thousand stomachs. There were one A thousand arms and one thousand eyes, and these seemed to spout out fire. O mighty-armed one! It is superior to Brahma, Narayana, Aindra, Agneya and Varuna weapons and is capable of countering all weapons. Or Govinda! In ancient times, Mahadeva sported around and with this single arrow, in an instant, consumed and reduced Tripura to ashes. There is no doubt that if it is released from Maheshvara's arms, in an instant, it consumes the entire universe and the three worlds, with their mobile and immobile objects. There is nothing in the world that can not be slain with it, even Brahma, Vishnu and the gods. Or are! I saw that extraordinary, wonderful and supreme weapon there.
My question is this, the roudra and pashupata weapons are the same weapon? If not, why does Shiva also refer to the pashupata weapon as roudra?