On only my 4th run of mythic Mogoshun Palace I got Elegon's mount. Its like I wont the lottery.
OTOH, Vanilla/TBC/Wrath era Stratholm over 200 times nad never got the mount, and I raided Karazhan weekly for 2 years plus a couple of dozen retro runs solo and never even seen Attumen's mount drop.
I've done Tempest Keep maybe 50 times as well and never seen Alar drop, so in the grand scheme of things it all balances out.