The Rechesh and Restlessness

What is the mysterious creature known as a Rechesh? At first glance, one might wonder how this relates to Bava Basra. However, Rav uses this enigmatic creature to clarify his position (152b). He asserts that a gift that employs two distinct methods to transfer it to the recipient is valid, describing it as a “super gift.” He characterizes this gift as one that rides on two “Richsi.” The Rashbam interprets this line to mean that the gift possesses two distinct powers, though he does not specify what a Rechesh is. Instead, he refers us to a verse in Megillat Esther. After Haman was hanged, Mordechai and Esther needed to send messengers to inform the populace about the change in the King’s decree. The verse states (8:10), “Letters were dispatched by mounted couriers riding horses, rochvei haRechesh.” The Rashbam opines that the Rechesh mentioned in our Gemara is the same one referenced in the pasuk.
The Ibn Ezra in Megila explains that rochvei haRechesh means “used in the King’s service.” However, this interpretation does not align with Rashbam’s view in Bava Basra. It appears from our Gemara that Rechesh refers to a type of animal, not merely an adjective. The Aruch suggests that the term Rechesh used by Rav denotes a horse, implying that the gift he discusses is particularly enhanced, much like someone riding on two horses. However, this interpretation does not seem to correlate with the pasuk in Megilla, as the verse mentions Rechesh after already explicitly referring to horses. This suggests that Rechesh and horses are two separate creatures. The Metzudas Dovid speculates that Rechesh may be either a subspecies of horse or an entirely different, unknown animal. An anonymous comment from the Mahari Kra, included with the sefer, proposes that Rechesh is an eight-legged horse.