Is there a specific such creature, with a name for it and description, in any lore or game?
In D&D 3/3.5e there is a creature called a Dragonnel (p.151-152 of the Draconomicon). While not strictly a half-horse/half-dragon it serves the purpose being essentially a winged steed that very much looks like a scaly horse with dragon wings. It also has a little bit more background then the templates below.
Pathfinder Bestiary 2 also has a creature called a Dragon Horse however this is not a draconic creature but merely a horse that flies. The Dragon Horse is also in the Tome of Horrors (Complete for Pathfinder rules, Revised(p.167) for D&D rules). It looks like the Bestiary 2 version is probably based partially off of this and the original Gary Gygax version.
In Dragon Magic, there is another dragon/horse hybrid that might work: the Drakkensteed. Best link I could find talking about it is here.
The Qilin or Kirin is a creature of Chinese mythology that has been described in various ways, one of which is a half-dragon, half-horse or half-deer.
The "Ki-rin" appears in the (A)D&D monster manuals as early as 1st edition AD&D's Monster Manual (1977), where it is depicted as a scaled, Chinese-dragon-headed, hoofed equine that flies but is wingless (much like Chinese dragons). In D&D they are very rare and very intelligent, and as likely to serve as mounts as Western unicorns, i.e., not very often at all.
sounds like something out of Gamma world O_O
Though, from some checks around... there is at least a drawing someone made based on... apparently, the same topic that popped up for a discussion. You can see the image here if you want something to start with. Also, from another page talking about a person who rides a half-dragon/half-horse
If the Valor d10 game was out, that could most likely be accomplished within 5 min for a monster make.
But I do agree from most people's perspective... how would that happen?