Regarding mention of the name of Rishabha .
The word Rishabha is used in Yajurveda for ram not for Tīrthaṅkaras. In Yajurveda Samhita 21.60 .According to some sources the actual name is Usabha and not Rishabha. The word Rishabha is used for ram many times in all four vedas.
सुपस्था s अद्य देवो वनस्पतिरभवदव्श्रीभ्यां छागेन सरस्वत्यै
मेषेणेन्द्राय ऋषभेणा क्षस्तान
मेदस्त: प्रति पचतागृभीषतावीवृधन्त
पुरोडाशैरपुरश्विना सरस्वतीन्द्र: सुमात्रा सुरासोमान् |
60 To-day the divine Vanaspatî has done good service to the Asvins
with a goat, to Sarasvatî with a ram, to Indra with a bull. They have
eaten these from the marrow onwards, they have accepted the cooked
viands, they have waxed strong with the rice-cakes. The Agnis,
Sarasvatî, and Indra have drunk the Surâ and Soma draughts.
According to Hinduism Rishabha is one of the twenty two avatars of Vishnu in the Bhagavata Purana.
Here is English Translation.
Regarding mention of the name of Ariṣṭanemi.
Ariṣṭanemi was one of the five sons of Vinatā and Kaśyapa, and a brother of the famous Garuḍa. Sometimes, the sage Kaśyapa himself has been known by this name.
The Pāṇḍava prince Sahadeva had assumed this name when he lived in Virāṭanagara, in disguise.
This whole mantra is from Yajurveda samhita 25.19 and .Here we can see the meaning of Ariṣṭanemi
स्वस्ति न इन्द्रो वृद्धश्रवाः स्वस्ति नः पूषा विश्ववेदाः ।
नस्ताक्षर्यो अरिष्टनेमिः स्वस्ति नो ब्रुहस्पतिर्दधातु ।| YV 25.19||
Swasti Na Indro Vriddhashravah Swasti Na Poosha Vishwa-Vedaah Swasti
Nastaaksharyo Arishtanemih Swasti No Brihaspatir-dadhaatu
May God Indra of Great fame bless us , May the Omniscient Poosha bless
us ,May the Protector Garuda bless us ,May Lord Brihaspati
Regarding mention of the name of Ajitanatha .
The wikipedia itself say that - "The Yajurveda mentions the name of Ajitanatha, but the meaning is not clear."
According to this site Ajitanatha is mentioned in Yajurveda 25, 19 and 9, 25
In conclusion there is no trace of tirthankaras in Yajurveda. only the similar names . And the meaning of those names in vedas are different.