Chromatic dragon (UnNetHack)
Chromatic dragons are a type of dragon introduced in UnNetHack. They share the same assortment of breath attacks and resistances as the Chromatic Dragon, the Cave(wo)man quest nemesis, who is renamed Tiamat in UnNetHack; however, they have a lower difficulty level and lack Tiamat's spellcasting and sting attacks.
Chromatic dragons cannot be chosen as a polymorph form, and their eggs cannot be wished for.
Although chromatic dragons are not considered unique monsters, they are not randomly generated, nor can they be genocided (or reverse genocided). They are only found at the bottom of the Dragon Caves, a new branch added to Gehennom, where several are guaranteed to appear in a lair.
Chromatic dragons are the source of chromatic dragon scale mail for many players; the scales and their mail confer the resistances of every variety of dragon scale mail, including petrification resistance (conferred by the 'stone dragon', which has a randomized name) and reflection. A wish for a statue of a figurine of a chromatic dragon is also worth considering, either to obtain scales in some manner or else keep as pets, and is the only way to generate them outside of the Dragon Caves; all statues are non-magical, so it is valid for a wish from a magic lamp.
As of UnNetHack 5.2.0, these resistances also include magic resistance, since omitting it was a bug, however any chromatic dragon-related wishes (including for statues or figurines) are now disallowed. 
In earlier versions, players could attempt to acquire chromatic dragon scales without entering the Dragon Caves by wishing for one or more chromatic dragon eggs; this option was eliminated as of development version r1580.
Fearsome dragons who are channeling the powers of lesser dragons
like their mother Tiamat. Legends tell of a far away realm where
these creatures can be found in large numbers.