I think while the Horses would have been interested in the Tomahawk as an example of what they could be facing in the future, and the state of Wolf/IS Clan tech and production. I don't think they had the infrastructure or inclination to devote the resources necessary to build their own Tomahawks. The Horses didn't exactly take a bunch of well populated, industrially developed worlds from the Wolves.
Also, while the Tomahawk was in advanced testing stages there were still kinks that needed ironed out before it could enter mass production, and the Horses may have felt they had bigger priorities than work out any remaining bugs and reverse engineer the Tomahawk in addition to building the Assault Omnimech factory necessary to start producing Tomahawks. Especially when Hellstars and Cygnuses were nearly just as effective, as well as cheaper and easier to produce.
By the time the Wolves move to the FWL and the Horses take most of the Wolf OZ, Weingarten has already been producing Tomahawk IIs for 50 years. So it makes more sense for them to just replace whatever the Wolves managed to take with them from their Weingarten factories and produce Tomahawk IIs.