ascii

Linux

for generating ascii on the fly you can use figlet
apt install figlet

figlet has a lot of styles you can use, you can run showfigfonts to list them
showfigfonts

figlet -f fontname input

more figlet fonts https://github.com/xero/figlet-fonts git clone it, move the fonts to /usr/share/figlet

nonsense

https://github.com/busyloop/lolcat apt-get install lolcat lolcat will generate rainbow colors, for bashscript or so...

you can generate ascii animals with cowsay package, also installs cowthinks
apt-get install cowsay

you can list animals with
cowsay -l

for single user you could add it to ~/.bashrc or motd cowsay -f eyes "Welcome $USER"

to make it more fun pipe things to it, fortune is a separate package to install fortune | cowsay -f animal