This is a description of Scylla, the sea monster, found in the Aeneid (3.426-428) :
Prima hominis facies et pulchro pectore uirgo
pube tenus, postrema immani corpore pistrix,
delphinum caudas utero commissa luporum.
I do not understand the last part : "delphinum caudas utero commissa luporum" and especially the accusative in caudas. Besides, if luporum is associated with utero, why is it plural ?
caudas must be the not so common accusative of respect (see also this question).
"[Scylla] commissa caudas utero" means literally Scylla is connected/joined to the belli with respect to the tails. i.e., the tails are joined to her belli of wolves. Perhaps better way to approcuh this Greek-styled accusative is to attache the adjective (usually p.p.p) to the accusative: so [volucres] perculsae corda -> volucrium corda perculsa (sunt) So here it means: [Scyllae] caudae commissae utero.
As for the second question of why luporum and not lupi, I would not read much into that. It does not strike me as an odd choice also in other languages. Or even it means that the belly itself is formed of wolves as seen in one translation (Unfortunately I'm too ignorant of Greek Mythology, and fortunately never seen her myself so to tell how Scylla really looks like).
It seems some viewed Scylla's belly indeed formed of multiple wolves as this pre-virgilian art shows (Wikimedia): 
. But again, even if luporum here is in the sense that wolf-like belly I would not rule out luporum as a valid option but it might be interesting to explore in a separate question perhaps.
