Here we've listed the Heterodontosaurians and other basal ornithischians which do not belong to any of the main groups of bird-hipped dinosaurs. Heterodontosaurus, Lesothosaurus, and (less-frequently) Scutellosaurus are the most common in dino-books; in older works, however, you'll more frequently read the name "Fabrosaurus".
Among basal Ornithischian dinosaurs, there were curious things. Heterodontosaurus, for example, might be renamed the boar-bird.
Living in Early Jurassic South Africa 190 million years ago, Heterodontosaurus superficially resembled the ornithopod Hypsilophodon with its slender, bipedal body, but was even smaller (1.20 m/4 ft long), more robust and with longer forelimbs. Discovered only in the sixties, its name means “lizard with different teeth”, and with reason: no other dinosaur had such a diversified dentition, with three kinds of teeth surprisingly similar to those found in mammals. The most noticeable are two pairs of canine-like “tusks” visible when the mouth closed, giving it a vaguely boar-like look; behind, molar-like teeth to grind up tough vegetation; in front of them, small peg-like teeth only on the tip of its upper jaw.
With this dentition, Heterodontosaurus was probably a mostly herbivorous omnivore, eating insects other than vegetation, while the tusks could have been used for display and/or competiton. Some scientists suspect only males did have the large canines, but there is no evidence. Other close relatives, like Abrictosaurus, are devoid of tusks: their skull could either pertain to females, or, more probably, to totally tusk-less species.
Heterodontosaurus is the prototype of its own ornithischian family, Heterodontosaurids. Once thought ornithopods or ancient relatives of ceratopsians and pachycephalosaurs, now they are regarded as very basal ornithischians. Despite their primitiveness, heterodontosaurs not only flourished in the Early Jurassic, but also managed to survive until the Late Jurassic and even the Early Cretaceous, with species such as the poorly-known Echinodon from England. Even smaller than Heterodontosaurus and with small tusks only in the upper jaws, Echinodon is known to science since the middle XIX century, but its classification as a heterodontosaurian has been confirmed only after the discovery of the namesake of the group.
The same thing happened to Geranosaurus and Lycorhinus, both found in South Africa at the start of the XX century, and both showing prominent tusks (the latter, because of this, was initially believed a non-dinosaurian therapsid like Cynognathus). Also South-African and Early-Jurassic, Abrictosaurus was found about in the same period of Heterodontosaurus; its name, "awake lizard", is actually ironical, because it was hypothized that this animal underwent long "hibernations" to survive the harsh desertical conditions — the same hyp was made about another contemporary basal ornithischian, Lesothosaurus (see below).
Some important dinosaur discoveries that have been made since the 2009 regard the heterodontosaurian Group. For example, Fruitadens lived in the Late Jurassic North America alongside the famous jurassic Stock Dinosaurs; in opposite to Echinodon, Fruitadens has tusks only in its lower jaw. With only two feet of length (the same size of Microraptor), Fruitadens and Echinodon are among the smallest bird-hipped dinosaurs ever discovered — if the smallest. But the most extraordinary recent discovery about the "boar-birds" is another: see at the bottom of the page.
Size doesn't matter (just for once): Lesothosaurus & Fabrosaurus
When talking about Ornithischians, we can find the same issues of Saurischians: in the Triassic/Early Jurassic they were all so-similar each other, it’s hard task to classify them accurately. Nonetheless, they are extremely important animals for scientists, no matter their often tiny size. Other than the heterodontosaurians, we have several other examples.
Lesothosaurus, Scutellosaurus and Pisanosaurus have traditionally been the most relevant. Lesothosaurus in particular was once considered the forerunner of all bird-hipped dinos, and thought not to belong to any great ornithischian group; recent research suggest it could be a very basal Thyreophoran, thus ancestor of Stegosaurs and Ankylosaurs. From Early Jurassic Southern Africa like Heterodontosaurus, its name derives from the Kingdom of Lesotho (a small South African enclave) where its remains were dug out in 1978. Merely 3 ft long, the bulk of a Compsognathus, Lesothosaurus seems not to have any specialization in its anatomy. Its mouth had simple teeth and small cheeks, its forelimbs short and five-digited, its hindlimbs apt for running. It was said that Lesothosaurus resembles a lizard more than any other dinosaur.
Fragmentary remains from the same location that have been named Fabrosaurus may be synonymous with Lesothosaurus; since they were named before Lesothosaurus, Fabrosaurus would be the valid name for this dinosaur. Other possible Lesothosaurus remains have been classified in 2005 in another genus, Stormbergia. In old textbooks, the "fabrosaur" was often shown as the prototypical basal ornithischian; since the 1980s, Lesothosaurus took over this role; now the recently-discovered Eocursor (see below) is going to become the new archetypical ur-ornithischian. Once, the "fabrosaurid" family was recognized by scientists as a catch-all grouping for all undetermined basal ornithischians, but modern cladistic science do not accept artificial assemblages like this, and "fabrosaurid" has disappeared in literature.
Armor or not-armor?: Scutellosaurus & Pisanosaurus
Scutellosaurus has traditionally been the most primitive thyreophoran. Discovered only in the 1980s, was also a small bipedal animal with a similar look, but slighty bigger, longer-tailed, more robustly-built than the lesothosaur, and with a light armor made by small bony plates placed in rows upon its torso, similar to that of the bigger Scelidosaurus. Like the scelidosaur, Scutellosaurus lived in Early Jurassic, but was found in Arizona, where the popular double-crested Dilophosaurus lived (and could have been that dinosaur's prey).
Also found in the last decades of the XX century, the Argentinian Pisanosaurus lived with the alleged “first theropods" Eoraptor and Herrerasaurus, and still remains the most ancient ornithischian known to science. Sadly, is known only from one incomplete fossil, but was arguably similar to Lesothosaurus in shape and size and with no armor like the latter. One curious thing is that some Triassic non-dinosaurian archosaurs were once considered basal ornithischians as well: Technosaurus from Texas is one example, sometimes mentioned as "the most ancient North American ornithischian". Its evocative name comes from the Texas Tech University; interestingly, another basal ornithischian, the European Emausaurus (known only from a skull) also derives its name from an university, the German EMAU. It is usually believed in the middle between Scutellosaurus and Scelidosaurus, but some think it's a very primitive stegosaurian.
Two great little discoveries: Eocursor & Tianyulong
Like the basal saurischians, basal ornithischians as a whole are known only since the 1960s, and still aren’t well-understood. So, every recent discover could be very significative. Eocursor and Tianyulong in particular, are fairly gaining more and more consideration in scientific field because of their objective importance.
Found in 2007, Eocursor (“dawn runner”) was discovered in South Africa like Heterodontosaurus and Lesothosaurus, and its name is cleary inspired from that of Eoraptor (“dawn robber”). Its relevance is due to the fact that it’s the only Triassic ornithischian known so far from a complete skeleton (while the Pisanosaurus one is only partial); this gives us precious information about the deepest ornithischian roots, and also could better explain the relationship between bird-hipped dinosaurs and the saurischians.
According to the most accepted classification, ornithischians are divided in two main lineages: Thyreophorans and Cerapods. The former are, as is known, Stegosaurs+Ankylosaurs+some basal forms (Scelidosaurus, Scutellosaurus). Cerapods include almost all the other ornithischians, furthermorely divided in Ornithopods (duckbills, Iguanodon, Hypsilophodon etc) and Marginocephalians (ceratopsians+pachycephalosaurs). Indeed, Cerapods is just a Portmanteau made of “Cera[topsian]” and “[Ornitho]pod”.
About Tianyulong (guess which country it comes from?): this is a heterodontosaurid from the Late Jurassic found in 2009 in the same Liaoning site from which the Jurassic troodont Anchiornis was discovered, Tianyulong, like the latter, has preserved some sort of proto-feathers around its body. The thing is, this is the first time that unequivocally feather-like structures have been found in a non-theropod dinosaur (not counting the quills of Psittacosaurus of course) see the useful notes about dinosaurs in general to understand the revolutionary implications of this discovery.