There is no guarantee of coat pattern when crossing two spotted horses; it can result in solid coloured as well as spotted foals; just as a spotted and a solid horse very often produce a spotted foal when bred to one another. 

If a foal appears to have been born solid, the characteristics above may start to appear (or be present) and may indicate that the horse/pony will 'colour out', and gain more spotted patterning, as it gets older.