You may have read an earlier version of the spell's text. In August 2017, the errata document for the Player's Handbook included a change to the find steed spell.
Originally its wording sounded more open ended:
Appearing in an unoccupied space within range, the steed takes on a form that you choose, such as a warhorse, a pony, a camel, an elk, or a mastiff.
Post-errata, the phrase 'such as' was removed, making those five creatures the only possible choices. (Barring DM permission, as the spell mentions.)
Appearing in an unoccupied space within range, the steed takes on a form that you choose: a warhorse, a pony, a camel, an elk, or a mastiff.
That errata came out just 3 months before the find greater steed spell was published in Xanathar's Guide to Everything, which had the same strict phrasing as the post-errata version of find steed.
Appearing in an unoccupied space within range, the spirit takes on a form you choose: a griffon, a pegasus, a peryton, a dire wolf, a rhinoceros, or a saber-toothed tiger.