I have a slight obsession with heraldry, which started about the time that I moved to England. This coincidence in timing is probably because certain English people care a great deal about their and/or other people’s rights to use coats of arms and whether they are using them correctly.
Who knows where all the rules came from, but suffice to say they are ridiculously complicated and prescribe every imaginable configuration. Are you a widow who’s also the heir to a title because you’re an only child? There’s a rule for that. Are you the ninth son of the first son of a living title bearer? There’s a rule for that.
There are rules for what colors you can use and when to use them, what animals go where and what position (attitude) they can be in. There’s even a special language (mostly English with healthy doses of French and Latin) used to describe exactly what the arms look like. The Royal Coat of Arms of the United Kingdom, for instance, looks like this:

And sounds like this:
Quarterly, first and fourth Gules three lions passant gardant in pale Or armed and langued Azure (for England), second quarter Or a lion rampant within a double tressure flory-counter-flory Gules (for Scotland), third quarter Azure a harp Or stringed Argent (for Ireland), the whole surrounded by the Garter; for a Crest, upon the Royal helm the imperial crown Proper, thereon a lion statant gardant Or imperially crowned Proper; Mantling Or and ermine; for Supporters, dexter a lion rampant gardant Or crowned as the Crest, sinister a unicorn Argent armed, crined and unguled Proper, gorged with a coronet Or composed of crosses patée and fleurs de lis a chain affixed thereto passing between the forelegs and reflexed over the back also Or. Motto ‘Dieu et mon Droit’ in the compartment below the shield, with the Union rose, shamrock and thistle engrafted on the same stem.
The rules and styles of heraldry also vary widely between the countries that employ them, but since I don’t live anyplace but here, I don’t really care (and the English system is quite enough to sort out!).
Here, look at these:

A quilled funeral hatchment with excessive mantling (the frilly bits adorning the helmet at the top of the shield), circa 1692. This would have been displayed on the coffin and later in the church of the deceased arm bearer.

The fantastic arms of the South Georgia and South Sandwich Islands, home to introduced reindeer (crest), and native fur seals and macaroni penguins (supporters).

The marshaled arms (multiple arms combined into one shield—in this case quartered ad nauseam) of George Nugent-Temple-Grenville, 1st Marquess of Buckingham. Aptly described by most scholars as “completely redonkulous,” this was presumably an exercise in grandiosity and not intended for everyday use.