For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
The colt of the donkey which carried our Lord on the Palm Sunday was untrained colt as evidenced by Mk 11:2. It is risky to ride an untrained animal; but the colt was submissive to our Lord who is the creator of animals among the creations.
I have read somewhere that the word "hosanna" means "save us now".
But, I think there is no merit in this view as it is inappropriate to impute this meaning when we see that the slogans were "Hosanna to the Son of David" and "Hosanna in the highest Heaven " It is submitted Hosanna is a word we use to praise our Lord similar to Hallelujah