Beelzebufo

Amphibious.  The Beelzebufo can breathe air and water

Standing Leap. The Beelzebufo’s long jump is up to 30 ft. and its high jump is up to 15 ft., with or without a running start.
Magic Resistance. The Beelzebufo has advantage on saving throws against spells and other magical effects.

Beelzebufo's Toxic Skin.  Any creature that grapples the Beezlebufo or otherwise comes into direct contact with the Beelzebufo's skin must succeed a DC 15 Constitution saving throw, taking 10 (2d10) poison damage and becoming poisoned on a failed save. If they fail the save by 5 or more they are also infected with the Sight Rot disease (pg 257 of the DMG). A creature no longer in direct contact with the Beelzebufo can repeat the saving throw at the end of each of their turns, ending the effects of being poisoned on itself on a success. 

A poisoned creature that starts its turn in direct contact or comes into direct contact with the Beelzebufo's skin takes 10 (2d10) poison damage and is forced repeat the saving throw. On a fail, they become 'Badly' poisoned increasing the duration of the poison by an additional 6 hours each time they fail the save. 'Badly' poisoned creature don't repeat a saving throw evening if they are no longer within direct contact of the Beezlebufo, and the poison must be removed by magical means, or waited out.

Casting a spell or other magical effect that would cure poison on the Beelzebufo will  cause its skin to fester and boil, instead dealing 15 (3d10) radiant damage, plus an additional 1d10 per spell level above 1st.

This poison can be harvested with a poisoners kit and a DC 20 Intelligence check.

Spellcasting. The Beelzebufo is a 10th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 14, +6 to hit with spell attacks). The Beelzebufo has following spells prepared:
Cantrips (at will): Poison Spray, Druidcraft, Vicious Mockery
1st level (3 slots): Command, Sleep, Bane, Ray of Sickness

2nd level (2 slots): Crown of Madness, Hold Person, Suggestion

3rd level (1 slots): Bestow Curse, Hypnotic Pattern

4th level (1 Slots): Blight, Compulsion

Multiattack. The Beezlebufo makes a melee attack and casts a cantrip, or makes two bite attacks. If the first bite attack hits and successfully grapples the creature, the Beezlebufo uses its Swallow on it instead.

Bite. Melee weapon Attack: +7 to hit, reach 5 ft., one target. Hit:  9 (1d10+4) bludgeoning damage.The target is grappled (escape 15 DC ) until this grapple ends, the target is restrained and the Beelzebufo can’t bite another target.

Crush. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 10 (1d12+4) bludgeoning damage. 

If Beelzebufo moves into a space occupied by at least one creature as part of the landing from a jumping movement while attacking in this manner, the Beelzebufo attempts to body slam any creatures occupying the space as part of the attack. A creature hit by this attack must make a DC 15 Dexterity save, taking an additional 12 (2d12 damage) bludgeoning damage on a failed save and half as much on a successful one. If a creature fails this save by 5 or more they are knocked prone and restrained (escape DC 15) underneath a portion of the Beezlebufo's body. If used in this manner, it can't be used as part of a multiattack.

Swallow. The Beelzebufo makes one bite attack against a medium or smaller target is it grappling. If the attack hits, the target is swallowed and the grapple ends. The swallowed target is blinded and restrained, it has total cover against attacks and other effects outside the Beelzebufo, and it take 10 (2d6+4) bludgeoning and 6 (2d6) acid damage at the start of each of the Beelzebufo’s turns. The Beelzebufo can only have one target swallowed at a time.

If the Beelzebufo takes 20 damage or more on a single turn from a creature inside it, the Beelzebufo must succeed on a DC 15 Constitution saving throw at the end of that turn or regurgitate the swallowed creature, which falls prone in a space within 10 feet of the Beelzebufo.

If the Beelzebufo dies, a swallowed creature is no longer restrained by it and can escape for the corpse using 10 feet of movement, exiting prone.