Vet is no longer actively lurking around here sadly, but believing what is said in the credits for Outbreak
"This mod is 100% FREE, not a single penny was made through donations, advertising"
"or anything similar! This is simply a collection of material gathered from other"
"free & publicly available websites translated into compatible versions for the"
"half-life 2 single player version of the source engine!! I take no credit, zero, NADA"
"for anything found in this mod!!!"
I think you are fair to use the models (and textures) as long as you give proper credit to ppl.
P.S.
I don't think Vet made them, but I've also failed to find anything on the internet regarding this specific zombie reskin, so who knows...