From The Merry Wives of Windsor (4.1). Having taught Latin in the past, this scene both hurts and humours my heart.

MISTRESS PAGE
Sir Hugh, my husband says my son profits nothing in
the world at his book. I pray you, ask him some
questions in his accidence.
SIR HUGH EVANS
Come hither, William; hold up your head; come.
MISTRESS PAGE
Come on, sirrah; hold up your head; answer your
master, be not afraid.
……..
SIR HUGH EVANS
What is your genitive case plural, William?
WILLIAM PAGE
Genitive case!
SIR HUGH EVANS
Ay.
WILLIAM PAGE
Genitive,–horum, harum, horum.
MISTRESS QUICKLY
Vengeance of Jenny’s case!* fie on her! never name
her, child, if she be a whore.
SIR HUGH EVANS
For shame, ‘oman.
MISTRESS QUICKLY
You do ill to teach the child such words: he
teaches him to hick and to hack**, which they’ll do
fast enough of themselves, and to call ‘horum:’*** fie upon you!
SIR HUGH EVANS
‘Oman, art thou lunatics? hast thou no
understandings for thy cases and the numbers of the
genders? Thou art as foolish Christian creatures as
I would desires.

* Quickly interprets the words genitive case as a gross allusion to the vagina (case) of a whore, since prostitutes were mostly known by diminutive nicknames.
** Quickly takes the nominative case of the pronoun to mean to hiccup (for drunkeness) and to go whoring.
*** fuck them; Quickly’s interpretation of the plural genitive horum.

A man can waste much time with an apparatus criticus and alternate readings. From Hamlet (1.1.39-43).

I could continue expanding references and commentary (and I wish I had the Andrews edition to add) but the short is that no one has any clear answer and everyone is willing to entertain a Shakespearean coinage – either by novel transference of meaning (using ‘harrow’ from the agricultural implement) or outright new verb formation. Accordingly, I make less an ass of myself in adding my own conjecture and connecting Q2’s ‘horrowes’ with Latin ‘horrere.’ Lewis and Short define here but I’ll cite the similar and possibly more relevant Dictionary of Medieval Latin from British Sources definition (with full usage illustrations in the link):

1 to bristle; b (fig.). c to stand up like spikes.
2 to shudder, shiver.
3 to be or become horrible, shocking, disgusting. 
4 to be or become horrified.

With that in mind, advance to Hamlet’s first encounter with the Ghost (1.4.55ish):

What may this mean,
That thou, dead corse, again in complete steel
Revisit’st thus the glimpses of the moon,
Making night hideous; and we fools of nature
So horridly to shake our disposition
With thoughts beyond the reaches of our souls?

The Arden editors gloss horridly thus:

horrendously (a stronger meaning than modern ‘horrid’, possibly with a glance at the literal meaning of Latin horridus, bristing or with hair standing on end: see 1.5.19-20).

Here I would add that the DMLBS entry for horridus includes the additional sense ‘causing horror, dreadful.’ And that, bearing in mind the etymological connection with horrere and the above ‘shudder, shiver’ sense of the word, one can posit a clear play in line 55’s ‘so horridly to shake.’

Now to the second half of Hamlet’s encounter with the Ghost:

But that I am forbid
To tell the secrets of my prison-house,
I could a tale unfold whose lightest word
Would harrow up thy soul, freeze thy young blood,
Make thy two eyes, like stars, start from their spheres,
Thy knotted and combined locks to part
And each particular hair to stand on end,
Like quills upon the fretful porpentine:

Here there is no issue with the textual history – harrow is the universal reading – so I would instead draw attention to the hair imagery, which we’ve seen above is latent in the first two appearances of the ghost and directly connects to the imagery of horrere.

My vanity’s proposal is twofold. First, the less defensible element, that the harrow of 1.5 contaminated the reading of horrowes in 1.1. The only reason for preferring harrow is deference to the OED – that it feels less bold to add a new sense to an existing word than create a new one altogether. Second, that the root senses of horrere at least lurk as background associations in all three appearances of the Ghost – most expressly in the first (by my reading ‘horrowes’), more subtly in the second (submerged in ‘horridly’ but activated by ‘shake’), and as a verbal echo harrow/horrow/horrere triggered by hair-standing imagery in the third. All accomplished by a man with little Latin.

And this is why I stick to extracts. Playful and pointless argumentation takes too long.

From Julius Caesar (3.1).

CASSIUS
Where is Antony?
TREBONIUS
Fled to his house amazed:
Men, wives and children stare, cry out and run
As it were doomsday.
BRUTUS
Fates, we will know your pleasures:
That we shall die, we know; ’tis but the time
And drawing days out, that men stand upon.
CASSIUS
Why, he that cuts off twenty years of life
Cuts off so many years of fearing death.
BRUTUS
Grant that, and then is death a benefit:
So are we Caesar’s friends, that have abridged
His time of fearing death. Stoop, Romans, stoop,
And let us bathe our hands in Caesar’s blood
Up to the elbows, and besmear our swords:
Then walk we forth, even to the market-place,
And, waving our red weapons o’er our heads,
Let’s all cry ‘Peace, freedom and liberty!’
CASSIUS
Stoop, then, and wash. How many ages hence
Shall this our lofty scene be acted over
In states unborn and accents yet unknown!
BRUTUS
How many times shall Caesar bleed in sport,
That now on Pompey’s basis lies along
No worthier than the dust!

I find this scene one of the most terrifying miniatures in all Shakespeare’s psychological portraiture. A single line pushes Brutus from a predictable – from the classical rhetoric perspective – over-reasoned justification (‘So are we Caesar’s friends, that have abridged / His time of fearing death’) to a surreal dissociative break (Stoop, Romans, stoop, / And let us bathe our hands in Caesar’s blood … waving our red weapons o’er our heads, / Let’s all cry ‘Peace, freedom and liberty!’). Cassius’ attempted realigning of the ‘lofty scene’ as future exemplar flops out as Brutus picks up the performative-iterative element only to ritualise the slaughter over the exemplar – “How many times shall Caesar bleed in sport.” Any possible value of the action is here submerged in the character’s inability to focus on anything but the blood. The bathing in blood is not in Plutarch or any other source.