If you're building IBG, don't bother with FH. The passive is NICE, but you already have 40% CDR, so Randuin's offers better stats overall.
Hecarim needs 40% CDR, even if it does put him over 40% CDR. Except for his Q with two stacks, he has LONG cooldowns. As long as you aren't building silly items like Morello's for CDR on Hecarim, it's fine.
No, I mean that with IBG (10% CDR), SV (20% CDR), and Lizard Elder (10% CDR), he already has 40% CDR. The 20% CDR from FH would be redundant.
IBG also gives him mana already, so the only useful stat from FH (aside from its Passive) is armor, which Randuin's provides, along with health, as well as a decent passive and an incredible active.
My point being, if you have that build, the 6th item is better as Randuin's than Frozen Heart.
MOAR CDR --- "Happiness is nature's way of telling human resources you're overpaid." - Catbert