The type line of a Magic card is printed directly below the illustration. It contains the card’s card type(s). It also contains the card’s subtype(s) and supertype(s), if applicable. The type line has a set height and width. Because of this, sometimes there is no room for additional creature types, although they would have been expected based on flavor.  This most often happens with legendary creatures.
From the Comprehensive Rules (Core Set 2019 (July 13, 2018))
- 205. Type Line
- 205.1. The type line is printed directly below the illustration. It contains the card’s card type(s). It also contains the card’s subtype(s) and supertype(s), if applicable.
- 205.1a Some effects set an object’s card type. In such cases, the new card type(s) replaces any existing card types. Counters, effects, and damage marked on the object remain with it, even if they are meaningless to the new card type. Similarly, when an effect sets one or more of an object’s subtypes, the new subtype(s) replaces any existing subtypes from the appropriate set (creature types, land types, artifact types, enchantment types, planeswalker types, or spell types). If an object’s card type is removed, the subtypes correlated with that card type will remain if they are also the subtypes of a card type the object currently has; otherwise, they are also removed for the entire time the object’s card type is removed. Removing an object’s subtype doesn’t affect its card types at all.
- 205.1b Some effects change an object’s card type, supertype, or subtype but specify that the object retains a prior card type, supertype, or subtype. In such cases, all the object’s prior card types, supertypes, and subtypes are retained. This rule applies to effects that use the phrase “in addition to its types” or that state that something is “still a [type, supertype, or subtype].” Some effects state that an object becomes an “artifact creature”; these effects also allow the object to retain all of its prior card types and subtypes.
Example: An ability reads, “All lands are 1/1 creatures that are still lands.” The affected lands now have two card types: creature and land. If there were any lands that were also artifacts before the ability’s effect applied to them, those lands would become “artifact land creatures,” not just “creatures,” or “land creatures.” The effect allows them to retain both the artifact and land card types. In addition, each land affected by the ability retains any land types and supertypes it had before the ability took effect.
Example: An ability reads, “All artifacts are 1/1 artifact creatures.” If a permanent is both an artifact and an enchantment, it will become an artifact enchantment creature.
- 205.2. Card Types
- 205.2a The card types are artifact, conspiracy, creature, enchantment, instant, land, phenomenon, plane, planeswalker, scheme, sorcery, tribal, and vanguard. See section 3, “Card Types.”
- 205.2b Some objects have more than one card type (for example, an artifact creature). Such objects satisfy the criteria for any effect that applies to any of their card types.
- 205.2c Tokens have card types even though they aren’t cards. The same is true of copies of spells and copies of cards.
- 205.3. Subtypes
- 205.3a A card can have one or more subtypes printed on its type line.
- 205.3b Subtypes of each card type except plane are always single words and are listed after a long dash. Each word after the dash is a separate subtype; such objects may have multiple types. Subtypes of planes are also listed after a long dash, but may be multiple words; all words after the dash are, collectively, a single subtype.
Example: “Basic Land — Mountain” means the card is a land with the subtype Mountain. “Creature — Goblin Wizard” means the card is a creature with the subtypes Goblin and Wizard. “Artifact — Equipment” means the card is an artifact with the subtype Equipment.
- 205.3c If a card with multiple card types has one or more subtypes, each subtype is correlated to its appropriate card type.
Example: Dryad Arbor’s type line says “Land Creature — Forest Dryad.” Forest is a land type, and Dryad is a creature type.
- 205.3d An object can’t gain a subtype that doesn’t correspond to one of that object’s types.
- 205.3e If an effect instructs a player to choose a subtype, that player must choose one, and only one, existing subtype, and the subtype must be for the appropriate card type. For example, the player can’t choose a land type if an instruction requires choosing a creature type.
Example: When choosing a creature type, “Merfolk” or “Wizard” is acceptable, but “Merfolk Wizard” is not. Words like “artifact,” “opponent,” “Swamp,” or “truck” can’t be chosen because they aren’t creature types.
- 205.3f Many cards were printed with subtypes that are now obsolete. Many cards have retroactively received subtypes. Use the Oracle card reference to determine what a card’s subtypes are. (See rule 108.1.)
- 205.3g Artifacts have their own unique set of subtypes; these subtypes are called artifact types. The artifact types are Clue, Contraption, Equipment (see rule 301.5), Fortification (see rule 301.6), Treasure, and Vehicle (see rule 301.7).
- 205.3h Enchantments have their own unique set of subtypes; these subtypes are called enchantment types. The enchantment types are Aura (see rule 303.4), Cartouche, Curse, Saga (see rule 714), and Shrine.
- 205.3i Lands have their own unique set of subtypes; these subtypes are called land types. The land types are Desert, Forest, Gate, Island, Lair, Locus, Mine, Mountain, Plains, Power-Plant, Swamp, Tower, and Urza’s.::: Of that list, Forest, Island, Mountain, Plains, and Swamp are the basic land types. See rule 305.6.
- 205.3j Planeswalkers have their own unique set of subtypes; these subtypes are called planeswalker types. The planeswalker types are Ajani, Angrath, Arlinn, Ashiok, Bolas, Chandra, Dack, Daretti, Domri, Dovin, Elspeth, Freyalise, Garruk, Gideon, Huatli, Jace, Jaya, Karn, Kaya, Kiora, Koth, Liliana, Nahiri, Narset, Nissa, Nixilis, Ral, Rowan, Saheeli, Samut, Sarkhan, Sorin, Tamiyo, Teferi, Tezzeret, Tibalt, Ugin, Venser, Vivien, Vraska, Will, Xenagos, Yanggu, and Yanling.
- 205.3k Instants and sorceries share their lists of subtypes; these subtypes are called spell types. The spell types are Arcane and Trap.
- 205.3m Creatures and tribals share their lists of subtypes; these subtypes are called creature types. The creature types are Advisor, Aetherborn, Ally, Angel, Antelope, Ape, Archer, Archon, Artificer, Assassin, Assembly-Worker, Atog, Aurochs, Avatar, Azra, Badger, Barbarian, Basilisk, Bat, Bear, Beast, Beeble, Berserker, Bird, Blinkmoth, Boar, Bringer, Brushwagg, Camarid, Camel, Caribou, Carrier, Cat, Centaur, Cephalid, Chimera, Citizen, Cleric, Cockatrice, Construct, Coward, Crab, Crocodile, Cyclops, Dauthi, Demon, Deserter, Devil, Dinosaur, Djinn, Dragon, Drake, Dreadnought, Drone, Druid, Dryad, Dwarf, Efreet, Egg, Elder, Eldrazi, Elemental, Elephant, Elf, Elk, Eye, Faerie, Ferret, Fish, Flagbearer, Fox, Frog, Fungus, Gargoyle, Germ, Giant, Gnome, Goat, Goblin, God, Golem, Gorgon, Graveborn, Gremlin, Griffin, Hag, Harpy, Hellion, Hippo, Hippogriff, Homarid, Homunculus, Horror, Horse, Hound, Human, Hydra, Hyena, Illusion, Imp, Incarnation, Insect, Jackal, Jellyfish, Juggernaut, Kavu, Kirin, Kithkin, Knight, Kobold, Kor, Kraken, Lamia, Lammasu, Leech, Leviathan, Lhurgoyf, Licid, Lizard, Manticore, Masticore, Mercenary, Merfolk, Metathran, Minion, Minotaur, Mole, Monger, Mongoose, Monk, Monkey, Moonfolk, Mutant, Myr, Mystic, Naga, Nautilus, Nephilim, Nightmare, Nightstalker, Ninja, Noggle, Nomad, Nymph, Octopus, Ogre, Ooze, Orb, Orc, Orgg, Ouphe, Ox, Oyster, Pangolin, Pegasus, Pentavite, Pest, Phelddagrif, Phoenix, Pilot, Pincher, Pirate, Plant, Praetor, Prism, Processor, Rabbit, Rat, Rebel, Reflection, Rhino, Rigger, Rogue, Sable, Salamander, Samurai, Sand, Saproling, Satyr, Scarecrow, Scion, Scorpion, Scout, Serf, Serpent, Servo, Shade, Shaman, Shapeshifter, Sheep, Siren, Skeleton, Slith, Sliver, Slug, Snake, Soldier, Soltari, Spawn, Specter, Spellshaper, Sphinx, Spider, Spike, Spirit, Splinter, Sponge, Squid, Squirrel, Starfish, Surrakar, Survivor, Tetravite, Thalakos, Thopter, Thrull, Treefolk, Trilobite, Triskelavite, Troll, Turtle, Unicorn, Vampire, Vedalken, Viashino, Volver, Wall, Warrior, Weird, Werewolf, Whale, Wizard, Wolf, Wolverine, Wombat, Worm, Wraith, Wurm, Yeti, Zombie, and Zubera.
- 205.3n Planes have their own unique set of subtypes; these subtypes are called planar types. The planar types are Alara, Arkhos, Azgol, Belenon, Bolas’s Meditation Realm, Dominaria, Equilor, Ergamon, Fabacin, Innistrad, Iquatana, Ir, Kaldheim, Kamigawa, Karsus, Kephalai, Kinshala, Kolbahan, Kyneth, Lorwyn, Luvion, Mercadia, Mirrodin, Moag, Mongseng, Muraganda, New Phyrexia, Phyrexia, Pyrulea, Rabiah, Rath, Ravnica, Regatha, Segovia, Serra’s Realm, Shadowmoor, Shandalar, Ulgrotha, Valla, Vryn, Wildfire, Xerex, and Zendikar.
- 205.3p Phenomenon cards, scheme cards, vanguard cards, and conspiracy cards have no subtypes.
- 205.4. Supertypes
From the glossary of the Comprehensive Rules (Core Set 2019 (July 13, 2018))
- Type Line
- Part of a card. The type line is printed directly below the illustration and contains the card’s card type(s), subtype(s), and/or supertype(s). See rule 205, “Type Line.”