cowthink: 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/bin/cowthink - from package: cowsay

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

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