cowsay: 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/cowsay - from package: cowsay

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

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