The rules state the following in terms of suitable mounts:
[The DM has] the final decision on what is or is not a suitable mount. At its most basic level, a mount should have the following characteristics:
Able and willing to carry its rider in a typical fashion. (A camel is able and willing. A tiger might be capable but may not be willing. A giant might be willing but not truly able.) At least one size category larger than the character. Also, a flying mount can carry no more than a light load aloft.
Emphasis mine. However, what are the consequences of trying to mount a creature of the same size or smaller than the character in question? Especially if the would-be mount otherwise does have the strength to carry the would-be rider?