"Saya mendengar seekor binatang" is not acceptable in bahasa. Mendengar is an adverb that cannot stand alone and should be supported by noun as in "mendengar suara/gosip/sesuatu.
The sentence does sound weird, but "seekor binatang" is a noun. However, I agree "Saya mendengar suara seekor binatang" might be better, although that sounds much more formal, as if it's from a poem.
'An' digunaka ketika subjeknya berawalan fokal(a, i, u, e,o) contoh : i eat an apple