r.horizon: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/lib64/grass-6.4.4/bin/r.horizon - from package: grass

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/lib/grass64/bin/r.horizon - from package: grass-core

You may install the required package with command apt-get install {package}

Error in other languages:
r.horizon: bevel nie gevind nie
r.horizon: comandă negăsită
r.horizon: comando não encontrado
r.horizon: commande introuvable
r.horizon: command not found
r.horizon: käsku ei ole
r.horizon: không tìm thấy lệnh
r.horizon: komanda nerasta
r.horizon: Kommando nicht gefunden.
r.horizon: kommandot finns inte
r.horizon: komut yok
r.horizon: nie znaleziono polecenia
r.horizon: níor aimsíodh an t-ordú
r.horizon: no se encontró la orden
r.horizon: no s'ha trobat l'ordre
r.horizon: opdracht niet gevonden
r.horizon: parancs nem található
r.horizon: perintah tidak ditemukan
r.horizon: príkaz nenájdený
r.horizon: příkaz nenalezen
r.horizon: команда не найдена
r.horizon: командата не е открита
r.horizon: 命令找不到
r.horizon: コマンドが見つかりません