Māheśvara mūrtis are numbered as 25 out of which Ekapāda trimūrti is also one. My father took this form during the destruction of the worlds where Viṣṇu and Brahmā merged with him. This definition of this mūrti is as follows.
रक्तवर्णः त्रिणेत्रश्च वरदाभयहस्तकः।
ऋज्वागतस्तथैकेन पादेनापि समन्वितः।
कृताञ्जलिपुटौ एकपादयुक्तौ च वा मतौ। (Uttara Kāmikāgama)
This form is red in colour and has three eyes. He possesses Varada, Abhaya, antelope and axe on his hands. He is adorned with Jaṭāmakuṭa. He is straight and has only one leg. On his right and left, Brahmā and Viṣṇu are projected with half body. They too have one leg and has Añjali in their hands.
This mūrti is called as Trimūrti since it has three principal deities. The chapter itself is called as Trimūrti sthāpana vidhi. Such Ekapāda mūrtis are found and identified. But there is a special news from the same āgama. It gives other options to install this form.
अथवा शिवलिङ्गस्य पार्श्वयोः नतगतौ तु तौ।
Or else the two deities can be installed on either sides of a Liṅga with bowed faces.
अथवा तौ पृथक्स्थाप्याः वेकविष्टरमास्थितौ।।
Or they can be installed separately on the same platform where the liṅga is installed.
अथवा मध्यमे लिङ्गं पृथगालयसंस्थितम्।
तस्य सव्येपि असव्ये च ब्रह्मविष्णू तथा मतौ।।
Or the Liṅga can be installed in a separate shrine in the middle and others can be installed in right and left to the shrine.
पृथग्धामस्थिता वा एकधामस्था वा त्रिमूर्तयः।
संश्लिष्टधामयुक्ता वा ब्रह्मविष्णुशिवा मताः।
पूर्वास्याः पश्चिमास्या वा परिवारादिसंयुताः।
भिन्नप्राकारगा वापि एकप्राकारसंस्थिताः।
नृत्तमूर्त्यादिदेवा वा स्थापनीयास्तु मध्यमे।
Or they can be installed in separate temples or in a same temple as Trimūrtis. They can be installed in a adjacent-attached shrines as Brahma-Viṣṇu-śivas. They can face east or west. Even Parivāra devatās can be installed. They can be installed in a same prākāra of in different prākāras. Or even Naṭarāja and other such forms can also be installed in the middle.
Keeping these facts in the mind, we can see the temples of pallavas. The Cave temple of Mahendra I from Maṇḍagapaṭṭu has three shrines attached together and the inscription therein refers the temple as Brahma-viṣṇu-maheśvara lakṣitāyatanam. This can be a clear evidence for the above said Ekapāda trimūrti concept. Other cave temples can be analyzed in this way. The extra shrines found in some cave temples like Māmaṇḍūr must be dedicated for Parivāras.
Even the Kailāsanātha temple, an excellent example for a structural temple, too has this concept. The Viṣṇu and Brahmā are installed in the Mālikā structure in both Rājasiṃheśvara and Mahendra Varmeśvara.
All these can be quoted as the examples of Ekapādamūrti concept.
Picture courtesy : Chira. Arvind Venkatraman