Statue

Facts

Statues are represented on screen by the ` character.

Their weight varies, depending on which monster it is a statue of.

Statue traps are indistinguishable from statues, but instead of being stone objects, they are magical, and will turn into the appropriate type of monster when you try to move into their square.

Statues may contain spellbooks; this is more likely on lower levels. In addition, statues left in bones files of deaths by cockatrice petrification may contain items.

Strategy

Statues can be turned into their monster types by casting Stone to flesh on them. The opposite transformation can be effected with a cockatrice corpse.

Nethack encyclopedia entry

Then at last he began to wonder why the lion was standing so
still - for it hadn't moved one inch since he first set eyes
on it.  Edmund now ventured a little nearer, still keeping in
the shadow of the arch as much as he could.  He now saw from
the way the lion was standing that it couldn't have been
looking at him at all.  ("But supposing it turns its head?"
thought Edmund.)  In fact it was staring at something else -
namely a little dwarf who stood with his back to it about
four feet away.  "Aha!" thought Edmund.  "When it springs at
the dwarf then will be my chance to escape."  But still the
lion never moved, nor did the dwarf.  And now at last Edmund
remembered what the others had said about the White Witch
turning people into stone.  Perhaps this was only a stone
lion.  And as soon as he had thought of that he noticed that
the lion's back and the top of its head were covered with
snow.  Of course it must be only a statue!
                [ The Lion, the Witch and the Wardrobe by C.S. Lewis ]