Here are the locations of Magatia's monsters, with the last part of the desert too :

Sand Rat :
· Sunset Road - Sahel 3
· Sunset Road - Sahel 2
· Sunset Road - The Desert of Serenity

Scorpion :
· Sunset Road - Sahel 2
· Sunset Road - The Desert of Dreams
· Sunset Road - Sahel 1

Cube Slime :
· Zenumist Research Institute - Lab - Unit 101
· Zenumist Research Institute - Lab - Unit 102

Dark Sand Dwarf :
· Sunset Road - The Desert of Serenity
· Sunset Road - The Desert of Dreams

Rumo :
· Zenumist Research Institute - Lab - Unit 101
· Zenumist Research Institute - Lab - Unit 102
· Zenumist Research Institute - Lab - Unit 103

Triple Rumo :
· Zenumist Research Institute - Lab - Unit 102
· Zenumist Research Institute - Lab - Unit 103

Iron Mutae :
· Alcadno Research Institute - Lab - Area B-3
· Alcadno Research Institute - Lab - Area A-3

Reinforced Iron Mutae :
· Alcadno Research Institute - Lab - Area A-3

Mithril Mutae :
· Alcadno Research Institute - Lab - Area A-1
· Alcadno Research Institute - Lab - Area B-1

Reinforced Mithril Mutae :
· Alcadno Research Institute - Lab - Area A-1

Roid :
· Alcadno Research Institute - Lab - Area C-1
· Alcadno Research Institute - Lab - Area C-2

Neo Huroid :
· Alcadno Research Institute - Lab - Area C-2
· Alcadno Research Institute - Lab - Area C-3

Homun :
· Zenumist Research Institute - Lab - Unit 201

Homunculus :
· Zenumist Research Institute - Lab - Unit 202
· Zenumist Research Institute - Lab - Unit 203

Homunscullo :
· Zenumist Research Institute - Lab - Unit 202

Site :
· Zenumist Research Institute - Lab - Unit 201
· Zenumist Research Institute - Lab - Unit 203
· Hidden Street - Authorized Personnel Only

D.Roy :
· Alcadno Research Institute - Lab - Area C-3


Source : Maple Gear