The recent discovery of 2D magnetic materials, which are compounds of transition metals (TM) with other elements, has opened new avenues for basic research on low-dimensional magnetism and potential applications in spintronics. To further explore new 2D magnets of pure TM is an interesting topic. Based on the fifirst-principles calculations, here a strategy of obtaining monolayer TM magnets, i.e., depositing TM atoms on graphyne (Gy), which has proper hexagonal hollow geometry, is proposed. It is found that a TM monolayer with perfect hexagonal geometry can be formed on Gy. The TM monolayer exhibits a wealth of physical properties in dependence of TM species, such as ferromagnetic and anti-ferromagnetic ground states, as well as intriguing semimetal and half-metal characteristics. It is also found that the half-metal characteristics make the monolayer TM have great potential applications in horizontal magnetic tunnel junction (MTJ) devices, where the tunneling magnetoresistance can reach as high as 850 000%. These results provide a new framework for obtaining 2D magnets with outstanding spintronic properties.  see https://onlinelibrary.wiley.com/doi/full/10.1002/adts.202100287