"He does not threaten his new dog."
Translation:Ní bhagraíonn sé ar a mhadra nua.
An Irish speaker could ask exactly the same question in reverse - "Why doesn't the English sentence include a preposition?".
English and Irish are different languages, and in this case, Irish uses a preposition where English doesn't. ar is necessary because the Irish for "threaten" is a phrasal verb bagair ar.
Fix on, Stick on, Call on, Pick on, Hold on, Move on, Get on, Hit on: We have so many phrases in English using a verb plus "on" without the "on" really meaning all that much, hopefully we will be able to take on the idea that some phrases in Irish may take ar even where it doesn't appear to be absolutely necessary.
madadh is a spelling that reflects dialect pronunciation, but doesn't occur in the standard dictionaries, even as a variant (in other words, madra is pronounced "madadh" in Connacht and Ulster, and those pronunciations are different, because of the way that adh is pronounced).
This non-standard spelling has been added as an acceptable alternative to some exercises, I think, but that would be by request on a per-exercise basis, not a global change.
The Foclóir Gaeilge Béarla does have an example of Chuir siad bagairt orm - "they threatened me", in the entry for cuir ar, but it doesn't use that construction in either the entry for bagair or bagairt.
So it looks like it's not completely unheard of, but isn't usually used that way.