Which sentence sounds disparaging? Or both?

I think the first one is more to the point. I would reword it to "Jack acts like a hound around Mary.".

Thank you.

Do you think whether both of the sentences are an insult for Jack?

Either sentence properly dictates its intention to censure Jack's bad behavior towards Mary.

I think both could be considered insults

both sentences are dogs

(1) *Jack is used to act like a hound after Mary.*
(2) *Jack is used to act like a dog after Mary.*

Both sentences are ungrammatical as they are written, Ori.

If you mean the used which carries the meaning "formerly/as a past habit" then there's no 'be' verb used:

(1) Jack [is] used to act like a hound [after] around Mary.
(2) Jack [is] used to act like a dog [after] around Mary.

If you mean "familiar with/comfortable with" then you have to use "to acting":

(1) Jack is used to acting like a hound [after] chasing Mary.
(2) Jack is used to acting like a dog [after] chasing Mary.

"Hound" has some additional connotations that make it more suitable in my opinion, but both would be insulting.

From the free dictionary for hound:

Definition two is what you are after here although I've seen it used as 3a also as in he is pursuing Mary romantically. As JTT pointed out, both sentences need some work with grammar.

Thank you all.