The 13th–14th c. Vaiṣṇava theologian Veṅkaṭanātha (also known as Vedānta Deśika) opened various chapters (called vāda) of his Śatadūṣaṇī with a different praise of Hayagrīva. Interestingly, they focus on different aspects of this complex God. The first one focuses on His being connected with the Veda and speech, the second on the latter connection only, the last two on Him as the supreme deity, while the middle one is a sort of threshold between Hayagrīva’s connection to knowledge and Hayagrīva as supreme deity. Accordingly, the translation of this maṅgala is particularly tricky.

viditam anuvadanto viśvam etad yathāvad vidadhati nigamāntāḥ kevalaṃ yanmayatvam |
aviditabahubhūmā nityam antarvidhattāṃ hayavaravadano ‘sau sannidhis sannidhiṃ naḥ ||)

The second part of the verse is relatively clear, although I am sure I am missing something in the equation of Hayagrīva with sannidhi:

Let He, as proximity*, with the face of a horse, whose opulence is not understood, take perpetually place close to us ||

The first part is less clear and the following translation is only tentative (comments are welcome):

The Upaniṣads, by repeating what has been understood, properly distribute this all [knowledge], which consists purely of Him |

Now, the tricky part is the echo between vidita/avidita and vidadhati/antarvidhattām. Given that the the first part of the verse refers to the Upaniṣads and the second part refers directly to Hayagrīva, the gist of the passage appears to lie in the idea that the Upaniṣads are an excellent device for gathering knowledge, but Hayagarīva surpasses all possible human knowledge.

*I would now read it as “Let he, the depository of good things” (the puṇya for this translation accrues to H.I.’s comment below).

For Hayagrīva in other Viśiṣṭādvaita Vedānta texts, see this post. For Hayagrīva in Vaiṣṇava temples, see here.