Bibek Debroy says in his Ramayana translation, 'Hanumat is a case in point, where Hanuman seemed to be too contrary to grammatical principles'.
What grammatical principle is that? Is it really Hanumat in the Sanskrit original?
In UTTARA KANDA of Valmiki Ramayana, all related words such as Hanumat, Hanumata, Hanumatah, Hanumataa, Hanumati and Hanumana etc. are mentioned based on the vibhakti:
Thereupon beholding the huge bodied Airavat and considering him as a bigger fruit Maruti ran towards him.
Belaboured by the thunder bolt of Indra Hanuman fell down and broke his left jaw.