dnd-5emonsters
I was wondering whether in the official published materials, or in Unearthed Arcana, there are any creatures that have multiple types (for instance, creatures that in their description it says they are both Fey and Beast).
I appreciate any examples of such creatures – or, if none exist, confirmation that creatures only have the one classification of monster type and that's it.
There are two different ways a race could be "official": it could be "official" in the sense of fully-tested and legal for sanctioned organized play campaigns, and it could be "official" in the sense of being published in 1st-party material from Wizards of the Coast. Which one is relevant depends on why you're looking for an "official" race.
BR: Basic Rules, Chapter 2: Races
SRD: System Reference Document, 5e, "Races" at p. 3-7
PHB: Player's Handbook, Chapter 2: Races
DMG: Dungeon Master's Guide, "Creating New Character Options" at p. 285-287
AI: Acquisitions Incorporated, "New Race: Verdan" at p. 72-74
EE: Elemental Evil Player's Companion, Chapter 1: Races
GGR: Guildmaster's Guide to Ravnica, p. 12-21
MToF: Mordenkainen's Tome of Foes, throughout Chapters 1-5
MOoT: Mythic Odesseys of Theros, Chapter 1 > Races
LR: Locathah Rising, an Extra Life fundraiser from WotC
OGA: One Grung Above, an Extra Life fundraiser from WotC
PotA: Princes of the Apocalypse, Appendix A: Genasi
PS:ZEN: Plane Shift: Zendikar, Races of Zendikar at p. 7
PS:INN: Plane Shift: Innistrad, Life on Innistrad at p. 7
PS:KLD: Plane Shift: Kaladesh, Races of Kaladesh at p. 15
PS:AKH: Plane Shift Amonkhet, Races of Amonkhet at p. 12
Note that Plane Shift occupies a strange shade-of-grey space: it's "compatible with" D&D, published by WotC, but written by the Magic: the Gathering team instead of the D&D team (though the author of all of them was a former D&D team member). Notably, it comes with a disclaimer substantially similar to the one provided with UA articles:
The game mechanics in this supplement are usable in your D&D campaign but are not fully tempered by playtests and design iterations. For these reasons, material in this supplement is not legal in D&D Organized Play events.
SCAG: Sword Coast Adventurer's Guide, Chapter 3: Races of the Realms
TP: The Tortle Package, an Extra Life fundraiser from WotC
UA: Unearthed Arcana articles, link and name specific article when listing
UA articles are presented on WotC's website with the following notice:
You can think of the material presented in this series as similar to the first wave of the fifth edition playtest. These game mechanics are in draft form, usable in your campaign but not fully tempered by playtests and design iterations. They are not officially part of the game. For these reasons, material in this column is not legal in D&D Organized Play events.
VOLO: Volo's Guide to Monsters, Chapter 2: Character Races
WGtE: Wayfinder's Guide to Eberron, Chapter 3: Races of Eberron
E:RftLW: Eberron: Rising from the Last War, Chapter 1: Character Creation
Note that Wayfinder's Guide to Eberron and Eberron: Rising from the Last War now redundantly feature most of the same new races, though the latter reprints the goblin, bugbear, and hobgoblin races, and includes a new version of the orc race. WGtE was originally published as a "living document", and was later updated with the versions of the included races as they appear in E:RftLW.
EGtW: Explorer's Guide to Wildemount, Chapter 4: Character Options, Races
Monster Manual, Hoard of the Dragon Queen, Rise of Tiamat, Tyranny of Dragons Player's Guide, Out of the Abyss, Rage of Demons Player's Guide, Curse of Strahd, Storm King's Thunder, Tales from the Yawning Portal, Xanathar's Guide to Everything, Waterdeep: Dragon Heist, and Dungeon of the Mad Mage all do not provide playable races. Tomb of Annihilation does provide one in its The Tortle Package charity supplement (the eponymous Tortle). (And Xanathar's Guide to Everything does provide racial feats, so those interested in racial options may want to take a gander at XGtE anyway.) Tasha's Cauldron of Everything contains a Custom Lineage option, which is not a race per se, but relevant to mention.
All of the above, plus:
Disclaimer: As always, consult your DM before playing one of these races: these races may not be for all players. Stop playing a race if you're not having fun doing so. Neither Wizards of the Coast nor RPG.SE are responsible for anything you do while playing one of these races. Please roll responsibly.
The limits of the action economy (1 movement, 1 action, 1 bonus action and 1 reaction per creature per round) mean that a single creature is always at a disadvantage in terms of the number of things that it can do with respect to a group of creatures (like a PC party). The mechanical purpose of legendary actions (lair or otherwise) is allow the creature to break these constraints. In effect, legendary actions are simply things that a creature can do out of sequence.
In terms of determining a creature's challenge rating, the fact that an action is a legendary one is irrelevant: it is just something that needs to be included with all its other actions/bonus actions when determining its damage output (for offensive CR) or effective AC (for defensive CR). For example, an Ancient White Dragon has a damage output of:
This averages out to \$ (49 \times \frac23) + (144 \times \frac13) + 51 + 40 = 175 \$ for an offensive CR of 22. Yes, I know it has a CR20 in the book - this is just one of the many CR anomalies in the Monster Manual.
In general, legendary actions are not needed for monsters with a CR of below 10 or so because the PCs cannot generally fully utilize the action economy themselves when these CR are suitable solo opponents. The problem that legendary actions were created to resolve doesn't really kick in until the PCs are getting an action, a bonus action and a reaction almost every round. If you want you can add legendary actions to low CR monsters and boost their CR a bit but its not really worthwhile.
As an aside, Legendary Resistance despite its name is not a legendary action.
Best Answer
There are currently no monsters with more than one creature type
The Monster Manual (page 7) includes a list of all creature types (cutting out the descriptions):
After a rather large amount of scrolling through the results pages on dndbeyond I have come to the conclusion that there do not exist any monsters with multiple creature types.
There are creatures, like the Quasit, that are shapechangers, or are a specific subset of a type (in this case a Quasit is a fiend, but more specifically, a demon); however, shapechanger is not a creature type in and of itself, nor is demon. Similarly a Swarm of Bats has the creature type Beast, it being a swarm doesn't change that.
There are also creatures like the Tanarukk, a fiend (demon) and an Orc, except Orc is not a creature type, Humanoid is, but the Tanarukk are not humanoids. The description of this creature includes the following:
Despite there not existing any monsters with multiple creature types, there are unpublished races with multiple creature types. For example, the Unearthed Arcana Centaurs and Minotaurs (pdf direct link) include the following feature:
Notably the later officially published versions of these races do not have multiple creature types. So outside of this Unearthed Arcana I do not know of any creatures (NPC or PC) that have multiple creature types. I also do not know any way of adding a creature type onto an already existing creature without removing the one they currently have.