This is a basic guide to help those who play the game to find beast keys. These keys are required to do something in the Lost Lands. I will be spoiler free about it. I will only mention where to find then with sourced information from Gamefaqs forums. WARNING: the chests for the beast keys will not show up on the map or mini map. The following is the information for the keys:

1: Woodroost, upper right corner through trees. Find the secret path.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3512406788
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3512405752

2: Astrum Forest Trail South: through bushes under higher stone bridge. Secret Path.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3512419667
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3512419690

3: Hoplard Moutains South: Break a Glove-stone (Requires Glove weapon power up) then wait until the screen turns dark and a bridge of ice is formed. Chest is across the ice floats.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3512427717
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3512427799
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3512427830

4: Desert Tombs: In one of the many quicksand sink holes. This one is next to the healing spring. Kill four mimics for the Beast Key chest.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3512453436
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3512453405
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3512453420

5: Urbantheria Wharf. Hard to miss. Can be found during a certain event also. Can be gotten later also in the game.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3512459337
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3512459308

6: Stahl Plains South: Starting with one of the single trees in the lower part of the map, touch it and watch where the birds fly. Head towards that direction until you find another tree. Repeat until they lead you to a brown bush to cut and reveal. Do not leave the map or the birds will reset themselfs.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3512464238
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3512464259

7: Oread Mountain: go through a secret path under a bridge and it'll be at the end.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3512552214
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3512552128

8: Festibane Trail West: Break apart any obstacle in the area (rocks, bushes, even status effecting flowers) and kill all enemies and spawners. Chest will pop up hidden through an obscured path (also blocked by breakable flowers) just across from the exit leading to Festibane.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3513273997
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3513274023

9: Grimgreed: Buy from Junkmonger (after doing a few of his quests) for 100,000 libra.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3513275915
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3513276135

10: This one is a real doozy as you need to talk to an NPC named Gabriel at every place he visits in order. For help in the non city areas, look up your map for an NPC dot, while in cities just talk to everyone until you find the person that gives you shards before leaving. The key will appear where the below screenshots show. It will only appear AFTER you talk to Gabriel in every town. If curious, Gabriel's home town, which you can find him in, is Urbantheria.

In order:
Woodroost, Astrum Forest Trail West, Hoplard, Aldo Desert, Stahl Noble's Quarter West, Festibane, Laxavia, Grimgreed Forest North, Despercia

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3513279877
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3513279852

This video by youtube user JaidynReiman may help individuals that require a more in depth view of how to get the keys:

I want to thank the information provided at the website GAMEFAQS for helping me find the relevant information. This also was provided by Gamefaq forum user ARCHSAGEBADGE for answering a question about this exact quest.

https://gamefaqs.gamespot.com/switch/368723-trinity-trigger/answers/628895-where-can-i-find-all-the-beast-keys

I also wish to thank JaidynReiman's youtube video for a in depth location of each key.