Ekaśṛṅga means "single-horned".
Definition: mfn. having but one horn, unicorn
Thus, Ekaśṛṅgatanu means single-horned one. The equation is thus in reference to Vishnu-nArAyaNa's Matsya vibhava which is single-horned.
According to Matsya Purana [looking for exact verse to cite]
Boat, Rope and Anchor— The king was also told that the boat which would come to him was the Ship of the Vedas (Veda navam), that he should stock there all the seeds of living creatures (Sattia hjani), that he should pilot the boat by a rope to be supplied by the divine Fish and tie it to its horn to be protected by the glory of the Divine Creator Things happened as was predicted and all the assurances given were fulfilled The rope was the Cosmic Serpent and the Fish to whose liom the boat was anchored in the midst of the watery flood was the Divine Creator himself
Reference for word meaning is also available at Wiktionary