There are a few significant differences between dragons in vanilla NetHack and dragons in SLASH'EM.
New dragon varieties
While shimmering dragon scale mail gives displacement, the dragons themselves do not have displacement in the sense that the displacer beast or a player wearing a cloak of displacement does. Rather, they have a lower armor class than the typical dragon, -8 instead of -4, and are thus harder to hit, simulating displacement. Shimmering dragons' breath attack is magic missiles, and they thus possess player-style magic resistance. Their corpses do not confer any intrinsics. They are also a deferred feature in vanilla NetHack.
Deep dragons have drain resistance and poison resistance, and their breath attack is poison gas. Their bite attack can also drain levels from their victims. Their corpses are poisonous, but have a 100% chance of giving poison resistance. Wearing deep dragon scale mail confers drain resistance, a property of considerably more importance in SLASH'EM than in vanilla, given the abundance of wands of draining.
Differences between NetHack dragons and SLASH'EM dragons
SLASH'EM baby dragons have a base level of 4 instead of 12. Also, unlike in NetHack, where baby dragons are never randomly generated except in aligned dungeon levels and branches (Oracle, Gnomish Mines, Sokoban, Medusa, Vlad's Tower), SLASH'EM randomly generates baby dragons on ordinary levels (with a frequency of "2"). This means that a SLASH'EM player will encounter baby dragons earlier and more frequently than a NetHack player. On the other hand, SLASH'EM adult dragons have a base level of 18 instead of 15, and many places that normally have guaranteed adults dragons in NetHack, such as Fort Ludios or The Castle, can now have baby dragons as well. As a result of the changed monster polymorph rules, increased difficulty of adult dragons, removal of most guaranteed adult dragons, and increased level difference between adult and baby forms, dragon scale mail is significantly harder to come across in SLASH'EM without using reverse genocide or a wish.
SLASH'EM dragons also have additional characteristics not present in NetHack. For example, most baby dragons (all of them except deep, silver, shimmering) hit as a +1 weapon, and most adult dragons (all of them except silver and shimmering) hit as a +3 weapon.
Interestingly, deep, blue, black, white, red, and orange adult dragons have a chance of turning traitor, while shimmering, gray, silver, green, and yellow ones do not. In other words, chaotic dragons can turn traitor, and lawful ones cannot, with one exception (orange).
Raising SLASHEM Baby dragons into adults
A good way to quickly evolve your pet baby dragon is to lay your dragon egg and hatch it before you reach the Rat King's level And have it kill the scores of mice and rats found there. You will find that your baby dragon can kill most of them in one shot without them even being able to retaliate. This strategy is very effective if you are playing as a doppelgänger and acquire polymorph control before reaching the Rat King's lair.
- See SLASH'EM 0.0.7E7F2/mondata.c#line80; this is how monsters gain intrinsic magic resistance, with baby gray dragons being a special case