You summon the spirit of a dead animal that takes a zombie form of one of the following beasts and appears in an unoccupied space that you can see within range. Choose one of the following options for what appears:

Stench Kow, Rothe, Ox, Giant Lizard, Variant Giant Lizard, Giant Spider, Giant Toad

The steed has the statistics of the chosen form, but is considered undead. 

Your steed serves you as a mount, both in combat and out, and you have an instinctive bond with it that allows you to fight as a seamless unit. While mounted on your steed, you can make any spell you cast that targets only you also target your steed. While Mounted, when you activate Symbiotic Entity your mount receives the same benefits for so long as you stay on it. (Rules reminder: Mounted Creatures can only take the Dash, Disengage, or Dodge Action)

Once given an order, the creature continues to follow it until its task is complete. It may receive orders verbally, somatically, or from a telepathic bond it forms with you. 

When the steed drops to 0 hit points, it disappears, leaving behind no physical form, and its spirit departs. You can also dismiss your steed at any time as an action, causing it to disappear. In either case, casting this spell again summons the same steed, restored to its hit point maximum.

You can't have more than one steed bonded by this spell at a time. As an action, you can release the steed from its bond at any time, causing it to disappear.