Nor is there a rule for horses getting tired I do believe.
Everything is in 40k. Imagine it, and it can be there. There are countless worlds in the galaxy, of which any could have evolved life in any way to produce any thing (That goes for technology too.). So, if someone wants a dragon, he can have a dragon. Won't be official, but he can. And the Exodites do have a form of Dragon. More along the lines of a Cold one from Fantasy, but they do have them (And they look real real nice, and why I'm considering having an Exodite army sooner or later).
Orkses are dum, but dey gotta have dem leadaz to lead dem, so da leadaz 'r all sm'rt 'n stuph. So dey 'rn't to be runnin' around on any ol' log. 's gotta be a piece o' dat hardwood log to be good enuf for da boyz. 'ard like dere 'eads.