cow: 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 Debian possible default full paths are:

/usr/bin/cow - from package: fl-cow

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

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