I've read a number of "mainstream" reviews, and other than referring to fantasy elements and obvious symbolism (especially the rock in the pool), none of them address the rabbit specifically. The blogosphere does, however. I have found three reviews that all seem to indicate that the rabbit symbolizes the life Gal is trying to leave behind.
From Jason Phereus:
Don isn’t alone in his cave under the swimming pool of course, he’s
also joined by the movie's symbol of death (drive), something that’s
best summed up by the term Giant Death Rabbit. The Rabbit menaces Gal
in a dream sequence, and in his later meeting with crime kingpin Teddy
Bass the sequence is partially replayed, reintegrating it from the
imaginary realm into the film's real-world frame brilliantly. In an
early sequence both Aitch and Gal’s surrogate son fail to kill the
rabbits they are hunting due to problems with their guns, and soon the
Rabbit has returned in monstrous form to symbolize the world that Gal
has left behind. The world of crime and its transgressive delights.
Don Logan knows who Gal really is, what drives animate him.
From CultureVulture:
Glazer's capitulations to visual excess are mercifully brief, notably
when Gal experiences visions of some sort of anthropomorphic entity
presumably representing his criminal past (it looks like a cross
between Sasquatch and a jackrabbit).
And from Musing from a SciFi Fanatic:
Don Logan is synonymous with the boogie man. He's even represented by
a hideous, monstrous jack rabbit creature, a symbol of death for
Winstone's character, a symbol of his past he cannot outrun. This is
juxtaposed by a scene earlier when the men are unable to kill a live
jack rabbit in the Spanish desert. It is symbolic of their fortunes or
lack thereof to come.
