One of my players is a dragonborn and wants to know if he can bite enemies. I didn't see anything in the rules about this, does anyone know how it works?
Unless otherwise specified, biting someone is an Unarmed Attack which does 1 pt of damage (+ Str bonus). Anyone can bite their enemies, and that's how much damage they do. The Dragonborn have their breath weapon, but no special bite attack.
The only PC race listed in official materials that has a special bite attack is the Lizardfolk, whose bite is given as 1d6 + Str bonus (Volo's Guide). So your player, if he really wants to bite people, could change his character race in his design to Lizardfolk.
Or, optionally, with your permission as the DM, he could play a variant Dragonborn that has a bite attack equivalent to the Lizardfolk, but has no breath weapon, to balance it out.
Dragonborn as a race doesn't have a special bite attack and any bite would be considered as an unarmed strike that causes 1 damage (plus ability modifier, usually STR).
A dragonborn with Dragon Hide feat (Xanathar's Guide to Everything) gives a natural attack with claws (hands) only.
You can grow retractable claws from the tips of your fingers. Extending or retracting the claws requires no action. The claws are natural weapons, which you can use to make unarmed strikes. If you hit with them, you deal slashing damage equal to 1d4 + your Strength modifier, instead of the normal bludgeoning damage for an unarmed strike.
The feat Tavern Brawler (PHB) increases all unarmed strikes to a d4 dice, this includes biting. It also grants proficiency with improvised weapons. If a dragonborn with this feat bites someone it has a chance to grapple the enemy (increased bite force and it won't let go, similar to a dog or shark?)
A monk's unarmed strike dice grows via Martial Arts feature, it also gives the option to us a bonus action for another unarmed strike if the normal attack was one as well.
When you use your attack action to use your monk weapon or an unarmed strike, you can use your bonus action to deal an additional unarmed strike, if you have not used your bonus action yet.
A dragonborn monk with the Tavern Brawler feat as such could
- a) Punch something in the face and then do a second unarmed strike described as a bite and try to grapple it
- b) Bite first to try to grapple it then do another unarmed strike (kick?)
- c) Use a monk weapon attack then attempt to bite the foe (grapple attempt) as second unarmed strike
Note that an unarmed strike is considered to cause bludgeoning damage unless specified otherwise. The Dragon Hide feat turns claw (hand) attacks into slashing damage (unless it hits with a fist or retracts the claws, by common sense), so while there is no RAW rule on a monk (or brawler) biting on an enemy discuss this the GM/players (I assume you're the GM in this case) to let a dragonborn's bite potentially cause piercing damage.
So, the short answer to the question is yes, every character can bite as an unarmed strike, but unless it has a racial trait Bite Attack (like the Lizardfolk race), a feat or class feature that increases the damage then the bite merely causes 1 damage (plus STR modifier).
Lizardfolk Bite Attack info in case you're interested:
Bite Attack: Your fanged maw is a natural weapon, which you can use to make unarmed strikes. If you hit with it, you deal piercing damage equal to 1d6 + your Strength modifier, instead of the bludgeoning damage normal for an unarmed strike.