Here is a photo of the Mount Boost icon, where you can clearly see that it is a Tauros:
Tauros cannot learn Agility or Extreme Speed, and it is not one of the five species of Pokemon (Arcanine, Rapidash, Garchomp, Latios, and Dragonite) that have Mount Boost automatically. One of the proposed solutions is to add Tauros to the list of Pokemon that get Mount Boost by default. This would be a simple solution, though Tauros might not make as much sense as the the five Pokemon that get it automatically, as I believe Tauros is known more for heavy tackles than fast tackles.
Another solution is to change the icon for Mount Boost to one of the Pokemon that automatically get the moveskill or to one of the Pokemon that can be taught it. Rapidash, Arcanine, and Latios are Pokemon that get Mount Boost by default that would be good picks for the Mount Boost icon. Of the Pokemon that can get Mount Boost by knowing Agility or Extreme Speed, I feel that Speed-form Deoxys, Suicune, Raikou, Entei, Rayquaza, Pidgeot, Dodrio, Scyther, Jolteon, Zapdos, Sceptile, Swellow, Sharpedo, Zebstrika, and Noivern would all be decent picks for the Mount Boost icon. However, this would likely be the most time-consuming solution as it would require designing new artwork or editing the existing one.
If the team does not want to add Tauros to the list of Pokemon that get Mount Boost automatically and don't want to change the artwork either, then another easy way to give Tauros access to Mount Boost is to pick a move that Tauros learns naturally to also unlock Mount Boost. Personally, I think that out of all the moves that Tauros can learn, High Horsepower (learned by TR) and Pursuit (learned by transfer) make the most sense to unlock Mount Boost. Frustration (learned by Alolan TM or transfer) and Wild Charge (learned by TR or Alolan TM) are also possibilities, though High Horsepower and Pursuit are probably better choices. I feel that this is the best solution to the problem. It doesn't require completely changing the Mount Boost artwork, and it doesn't require adding a Pokemon to the list of Pokemon who get Mount Boost automatically who is obviously only on the list so that the Mount Boost artwork isn't spreading misinformation. Also, Agility is a bad move to waste a moveslot on for most Pokemon and Extreme Speed is a move that not many Pokemon can learn, so making High Horsepower or Pursuit unlock the Mount Boost moveskill as well would be helpful.
In summary, these are my three proposed solutions to pick from:
- Add Tauros to the list of Pokemon species that automatically get Mount Boost. This would be an easy solution, though I understand if it is not picked as Tauros doesn't seem like the type of Pokemon that goes zoom as much as the five species that currently get Mount Boost by default like Rapidash and Latios.
- Change the Mount Boost icon to depict a player riding a Pokemon who can get Mount Boost legitimately. This would be an option if the team feels that it doesn't make sense for Tauros to have Mount Boost, but this option would require discarding the current art and making a new Mount Boost icon design.
- Have a move Tauros can legally learn also unlock the Mount Boost moveskill. High Horsepower or Pursuit are my recommendations. Frustration or Wild Charge are also options. This is the recommended approach, as it subtly gives Tauros access to Mount Boost at the same time as expanding Mount Boost's availability.
It just doesn't make sense for the Mount Boost icon to show a Tauros when Tauros can't get Mount Boost without cheating.