Originally, it was vertical, like:
\/
/\
\/
It was a symbol drawn in the sand/dirt in non-Christian times by those illegal Christians as a way to identify each other's
religion without being fed to the lions. The
two vertical arcs indicate a woman's anus and vagina as a referance to fertility, fertility of the
faith, the fertile
faith, or something like that. That is how early Christians told each other they were, one would start the arc, the other would finish the other side if they were too. This shape was chosen because it was simple and to a passing Roman/other enemy law official would be nothing particularly special.
Quite how it was bastardised into the horizontal <>< fish is beyond me, but, hey, it's a 2000 year old
religion, impossible to stay uncorrupted that long.