Critters III! Now also in 5e!

[Metastachydium]

Main Lobster

Is the 3.5 version amphibious?

Homarus regium

That's perfectly grammatical and sounds badass! It also means 'the lobster of kings', though; is that intentional?

Ram

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Ram (Ex): The Ram adds an additional 1d4 damage to it's head butt attack for every 10 ft. it moves while attacking during a charge (maximum of +10d4).

Well, damn. A +16 to hit dealing up to 12d4+2d6+4 damage. That's one hard punch to shrug off for a level 5 PC!

Bore

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Aura of Boredom (Su): Anything within 60' of the Bore must make a DC 19 Willpower Save (Save DC is Charisma Based) or begin to suffer from terminal boredom (opponents who Save successfully are immune to this Bores Aura for 24 hours). Anyone failing the Save must make another Willpower Save (same DC, +1 cumulative for each round within range of the Bores Aura) to perform any Full Round or Standard Action, or anything requiring a Concentration check. It must also Save if it wishes to use any class abilities based on emotions (for example Rage, Frenzy, or Wild Surge). Crushing apathy also causes victims to lose their Dexterity Bonus to Armor Class as long as they're under the effect of this ability. The effects of the Aura last as long as the Bore is within range. This is a Mind-Affecting Compulsion.

Sweet! I like it.

Sus taedium

More like Sus taediosus. Boredom pig is perfectly good English, of course, but in Latin the adjective would be the better pick.

Ghost Frog
Diminutive Undead (Incorporeal)
Hit Dice: 1d12 (6 hp)
Initiative: +2
Speed: Fly 10 ft. (2 squares), Perfect

I don't think flight speeds are ever measured in squares, regardless whether there are other movement speeds or not.

Attack: Nervously hesitant touch +6 melee touch (1d4 negative energy damage)

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Special Attacks: Corrupting Touch

So, what's this ability actually called?

Skills: Hide +20, Listen +4, Move Silently +10, Spot +10

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Skills: +8 Racial Bonus to Hide, Move Silently, and Spot checks.

Hide should be +22 (+2 DEX, +8 racial, +12 size). With 4 skill points per level and only a -2 for INT, it should also have 4 more skill points to spend.

Manifestation (Su): Every ghost has this ability. A ghost dwells on the Ethereal Plane and, as an ethereal creature, it cannot affect or be affected by anything in the material world. When a ghost manifests, it partly enters the Material Plane and becomes visible but incorporeal on the Material Plane. A manifested ghost can be harmed only by other incorporeal creatures, magic weapons, or spells, with a 50% chance to ignore any damage from a corporeal source. A manifested ghost can pass through solid objects at will, and its own attacks pass through armor. A manifested ghost always moves silently. A manifested ghost can strike with its touch attack or with a ghost touch weapon (see Ghostly Equipment, below). A manifested ghost remains partially on the Ethereal Plane, where is it not incorporeal. A manifested ghost can be attacked by opponents on either the Material Plane or the Ethereal Plane. The ghostÂ’s incorporeality helps protect it from foes on the Material Plane, but not from foes on the Ethereal Plane.

When a spellcasting ghost is not manifested and is on the Ethereal Plane, its spells cannot affect targets on the Material Plane, but they work normally against ethereal targets. When a spellcasting ghost manifests, its spells continue to affect ethereal targets and can affect targets on the Material Plane normally unless the spells rely on touch. A manifested ghostÂ’s touch spells donÂ’t work on nonethereal targets.

A ghost has two home planes, the Material Plane and the Ethereal Plane. It is not considered extraplanar when on either of these planes.

Rejuvenation (Su): In most cases, itÂ’s difficult to destroy a ghost through simple combat: The "destroyed" spirit will often restore itself in 2d4 days. Even the most powerful spells are usually only temporary solutions. A ghost that would otherwise be destroyed returns to its old haunts with a successful level check (1d20 + ghostÂ’s HD) against DC 16. As a rule, the only way to get rid of a ghost for sure is to determine the reason for its existence and set right whatever prevents it from resting in peace. The exact means varies with each spirit and may require a good deal of research.

"Ghost" should read "ghost frog" throughout. Same stands for the swarm.

Heleophryne depressa

A nice one again, but I'm not sure it's a good fit. Depressa can mean a lot of things ('low', 'flattened (both lit. and fig.)' and worse yet, 'muffled'). What were you intending to convey?