These symbols are called "wild" symbols and come in a variety of shapes, including stacked, sticky or expanding.
https://www.ww2.site/forum/viewtopic.php?t=393679
http://myweed.rocks/viewtopic.php?t=67246
https://gravatar.com/kittyjoyfullyd939ac1abc
http://www.lada-xray.net/showthread.php?p=312381#post312381
http://forum.animogen.com/viewtopic.php?f=76&t=1587406
http://60.250.226.63/ipkb/viewtopic.php?f=41&t=1934786
http://mirosovak.edoras.cz/viewtopic.php?f=3&t=531576
https://planeswalkers.org/viewtopic.php?t=60064
https://planeswalkers.org/viewtopic.php?t=37038
https://nb.gravatar.com/fornadisfragcorac
https://ufr-2020.front5.picapo.net/forums/viewtopic.php?pid=577439
http://60.250.226.63/ipkb/viewtopic.php?f=41&t=1974618
http://1.s.2.free.fr/viewtopic.php?f=18&t=572684
https://catrachosporelmundo.com/index.php?topic=29257.new#new
https://www.ww2.site/forum/viewtopic.php?t=259513
http://47.88.101.141/forum.php?mod=viewthread&tid=417251&extra=
https://muchroom.ng/index.php?topic=29524.new#new
https://planeswalkers.org/viewtopic.php?t=60574