  New surveys such as ESA's Euclid mission are planned to map with
unprecedented precision the large-scale structure of the Universe by measuring
the 3D positions of tens of millions of galaxies. It is necessary to develop
theoretically modelled galaxy catalogues to estimate the expected performance
and to optimise the analysis strategy of these surveys. We populate two pairs
of (1 Gpc/h)^3 volume dark-matter-only simulations from the UNIT project with
galaxies using the SAGE semi-analytic model of galaxy formation, coupled to the
photoionisation model get_emlines to estimate their H_alpha emission. These
catalogues represent a unique suite that includes galaxy formation physics and
- thanks to the fixed-pair technique used - an effective volume of ~(5
Gpc/h)^3, which is several times larger than the Euclid survey. We present the
performance of these data and create five additional emission-line galaxy (ELG)
catalogues by applying a dust attenuation model as well as adjusting the flux
threshold as a function of redshift in order to reproduce Euclid-forecast dN/dz
values. As a first application, we study the abundance and clustering of those
model H_alpha ELGs: for scales greater than ~5 Mpc/h, we find a
scale-independent bias with a value of $b\sim 1$ at redshift z ~ 0.5, that can
increase nearly linearly to $b\sim 4$ at $z\sim 2$, depending on the ELG
catalogue. Model galaxy properties, including their emission-line fluxes (with
and without dust extinction) are publicly available.
