A grenadier (derived from the word grenade) was originally a specialized soldier, first established as a distinct role in the mid to late 17th century, for the throwing of grenades and sometimes assault operations. At this time grenadiers were chosen from the strongest and largest soldiers. By the nineteenth century, the throwing of grenades was no longer relevant, but grenadiers were still chosen for being the most physically powerful soldiers and would lead assaults in the field of battle. Grenadiers would also often lead the storming of breaches in siege warfare, although this role was more usually fulfilled by all-arm units of volunteers called forlorn hopes, and might also be fulfilled by sappers or pioneers.

In the nineteenth century, certain countries such as France and Argentina established units of "Horse Grenadiers". Like their infantry grenadier counterparts, these horse-mounted soldiers were chosen for their size and strength (i.e. heavy cavalry).

Source: Wikipedia